by Ardea Skybreak
Revolutionary Worker #1183, January 19, 2003, posted at http://rwor.org
While there is still much that we will no doubt learn in coming years which will help us to more fully understand the complex history of hominid evolution, the basic story of our origins can be roughly summarized as follows:
All human beings alive today belong to the species Homo sapiens , which is part of the family Hominidae (the "hominids"), the biological family which includes humans, the last remaining species of African apes (gorillas and chimpanzees), and the Asian apes (orangutans, to which we are not that closely related). Human beings are not descended directly from chimpanzees or gorillas, but millions of years ago we did share a common ancestor species--one of the many species of apes that had earlier evolved on the African continent, and which was most likely a tree-dwelling, fruit- and leaf-eating species of the African forests, similar in many ways to today's gorillas and chimpanzees. Chimpanzees and human beings today are still so closely related that as much as 98-99% of our genetic DNA blueprint is the same!
Molecular biologists can compare human and chimp DNA to calculate the approximate time when our evolutionary line and the chimpanzee evolutionary line "split off" (or "diverged") from the species of apes that was the ancestor of both lines. Through this technique we now know that the initial split (or "branching event") took place a bit more than 5 million years ago. One line would eventually lead to the modern-day chimpanzee species.
The other line would eventually bring forth the modern human species. This particular branch of hominids started off with the evolution of a radically new feature--bipedalism. Though these early hominid ancestors of ours were still very ape-like in many respects, they were able to stand upright and walk on two legs.
The bipedal hominids were evidently quite successful: they spread and eventually spun off a number of additional species (each with somewhat different characteristics, but all of which could stand upright and walk on two legs). The bipedal hominid line produced a great variety of upright-walking species over the next few million years. Some of these bipedal hominid species were direct-line ancestors of our own modern human species, and some were more like separate branches of the same bipedal family tree, representing a number of alternative evolutionary pathways. Many of these species were successful, maintaining themselves as living species for hundreds of thousands of years or more, and some of them produced their own descendant species, but they all eventually became extinct.
We know that our modern human species, Homo sapiens,is the "youngest" of all the hominid species, having split off from its immediately preceding ancestor species only about 200,000 years ago. Today, all the other hominid species are gone. But as recently as 40,000 years ago, there were still at least two, or possibly three, different hominid species running around. They included:
1) Our own species, Homo sapiens,which had evolved in Africa about 200,000 years ago and started spreading to different parts of the world starting about 50,000 years ago.
2) Homo neanderthalensis (the Neanderthals) in Europe and the Middle East, which modern molecular analysis indicates was a different species of human. Homo neanderthalensis is thought to have last shared a common hominid ancestor with Homo sapiens about 600,000 years ago.* We know that populations of Homo erectus started migrating out of Africa more than a million years ago. The Homo neanderthalensis species in Europe and the Middle East is thought to have evolved out of these early Homo erectus migrants. When modern Homo sapiens evolved a bit later on (around 200,000 years ago) out of some of the African descendants of Homo erectus , it quickly spread and started its own out-of-Africa migration about 50,000 years ago. As it entered the Middle East and Europe it overlapped (for thousands of years) with populations of the Neanderthal species. We don't yet know much about how these two human species might have interacted. We do know that, while the Neanderthals had all sorts of stone tools, the Homo sapiens populations had more "advanced" stone tools which were more complex in both conceptual design and technical execution. Some of these sapiens tools have been found in Neanderthal archaeological sites, suggesting that at least some of the Neanderthals may have been trying to adopt this more advanced technology. We don't know whether the Neanderthal populations, on their own, encountered some environmental conditions they could no longer adapt to, or whether Homo sapiens populations had a more direct role in Neanderthal extinction (perhaps by outcompeting Neanderthals for food or other resources or maybe even by attacking them). But we do know that the Homo sapiens species had, one way or another, totally "replaced" the Neanderthal species by about 35,000 years ago.
3) Homo erectus descendants in Southern and Eastern Asia: These were also evolutionary descendants of those early populations of Homo erectus that had started to migrate out of Africa more than a million years ago, long before Homo sapiens had even evolved. We know that they had made it all the way to Southern and Eastern Asia (including China and Java). We also know from the fossil record that, like the African Homo erectus from which they were derived, they made a variety of stone tools and used fire. They managed to persist in Asia for hundreds of thousands of years, until as recently as about 30,000 years ago. We don't yet know whether Homo sapiens interacted with these Asian Homo erectus species, or what the nature of these interactions might have been, but we do know that the last of the Asian Homo erectus descendants became extinct around the time modern Homo sapiens began to spread into those regions.
So, having begun its own migrations out of Africa only around 50,000 years ago, our modern human species Homo sapiens eventually replaced all other human species everywhere it went. By 35,000 years ago we were the only ones left.
As we have seen, a "bush-like" pattern of multiple species--and a succession of speciation and extinction events--characterizes the overall history of our hominid line. This is a pattern commonly found in the evolution of biological species more generally: a new species often seems to get its start when a significant evolutionary "novelty" (such as the emergence of bipedalism in a line of apes) appears in a small population that has become reproductively isolated from its ancestral stock. If the new species doesn't go extinct right away, it then tends to undergo what is called an adaptive radiation:populations increase and spread into different locales, and there often start to spin off a number of additional descendant species in one or more waves of further evolutionary diversification. It is thought by many that such multiple speciation events are especially likely to take place at times of significant environmental change and disruptions.
In time, however, the rate of further speciations in any new evolutionary line tends to settle down, and a reduction is seen in the rate at which new species are produced. The analogy often given is that a new evolutionary bush at first spreads out, and for a while gets bushier and bushier, but then it eventually starts to get trimmed back through species extinctions.
This evolutionary pattern, which has been observed in the evolution of different plant and animal lines, is also the pattern of our own hominid evolution: at what was apparently something of a high point of hominid species diversification (in the period roughly between 2 and 3 million years ago) there lived as many as a half dozen or more different hominid species, including some of the last of the so-called "gracile" Australopithecines, some of the so-called "robust" Australopithecines and two or three early species in our own genus Homo . But by now this diverse evolutionary "bush" has been whittled down, and there is only one hominid species, our own species, Homo sapiens.
It is not easy to accurately sort out all the different degrees of relationship between different hominid species and figure out their distinguishing characteristics just from their fossil remains, but some interesting general patterns seem to be emerging. It seems pretty clear that, from our perspective at least, there were two particularly significant evolutionary junctures in the development of the hominid evolutionary "bush": First,the initial speciation event which marked the very beginning of the hominid line--the emergence of the very first bipedal hominids out of a line of African apes, an event which took place somewhere in the range of 5 to 10 million years ago.
And second,the tremendous increase in brain size and related capabilities which marked the emergence of the first hominid species with that characteristically human, but not ape-like, pattern of "slowed down" biological development, which results in infants being born essentially very undeveloped and requiring prolonged periods of parental care, but with the added "benefit" (from our perspective) of having brains which can continue to greatly increase in size and keep developing long after birth.This crucial change (which is associated with a greatly increased capacity to learn , much more than would have been possible previously) marked what I think of as the " second big leap" in hominid evolution--the one which really distinguished the newly emerging genus Homo from the variety of preceding Australopithecine hominids. And it was a change which seems to have come "packaged" with a bunch of other important anatomical and developmental changes, all of which made these hominids overall less like bipedal apes and more like modern humans, including: taller bodies with proportionally shorter arms and longer legs; flatter faces and more rounded skulls; changes in the size, shape, growth and development of the teeth; much less sexual dimorphism (much less difference in size between males and females than in earlier species); and an apparent shift in the position of the larynx (or voice box) to a lower position in the throat-- a change which allows humans to make many more vocal sounds than modern-day apes (and probably many more than early hominids). This shift in the position of the larynx, along with the significant post-natal brain development characteristic of the later hominids, could have been key to the development of a fuller human language , with all the resulting implications this would have for enhanced social communication and coordination.
A Possible Environmental Connection
Could environmental changes have somehow spurred on the emergence of bipedalism and the later emergence of increase in brain size, etc., in the hominid line? In examining this question, it is important to remember that an environmental change never directly "causes" an evolutionary shift to take place--that's just not how biological evolution works. But it is the case that an environmental change can sometimes dramatically change the local conditions in which populations of living plants and animals happen to live. In such cases, if an evolutionary innovation just happens to occur in a plant or animal line (through the usual random processes of genetic reshufflings, etc.) and if that randomly occurring genetic modification just happens to provide some kind of reproductive advantage to individuals having this new feature who are now living under these new environmental conditions, then it is likely that the new evolutionary feature will spread by natural selection. Under certain conditions (including sufficient reproductive isolation from the parental population), and especially if the evolutionary modification is sufficiently significant, the emergence and spread of the new feature (something like the emergence of upright walking in a line of tree-swinging apes) may be enough to spin off a whole new species.
It is very interesting to note that both of the periods when the two most significant modifications in hominid evolution took place were apparently also periods of major environmental change in large parts of Eastern Africa where the relevant hominid fossils have been found: First, in the period between 5 and 10 million years ago when bipedalism is thought to have emerged, there was a global cooling pattern and some active geological uplifting and fracturing on the African continent , which apparently caused a general drying trend in East Africa and a partial breakup of a relatively wide-ranging and uniform belt of African forests. The environment in East Africa apparently became much more "patchy," with areas of new mixed woodland savannas (consisting of more open areas dotted with clumps of trees) appearing where there once had been only fairly unbroken forests. It has been suggested that natural selection might well have encouraged the spread of upright walking, once it emerged in some population of forest apes, if it allowed these new oddball apes to expand their range and gain access to food and other resources in some of these new environments, where trees would have been spaced further apart from each other, and in which some of the traditional forest food supplies might have been drying up. Bipedalism could have been an advantage in these situations even if it were the case, as now seems likely, that the early bipedal species continued to spend much of their time moving about in trees and retreated to them for safety and shelter. Their new upright anatomies would almost certainly have facilitated their moving between separated clumps of trees in the new mixed woodland savannas. And while these earliest hominids did not make stone tools--and may not even have made any significant use of unmodified natural materials as tools, at least for some time--the fact that their hands would not have been needed for locomotion means that they could likely have covered greater distances on the ground, and it opens up at least the possibility that they could have begun to make greater use of their hands for purposes such as digging up edible roots and carrying stores of food over longer distances. All this, in turn, could have fostered increases in overall nutrition, boosted population numbers, facilitated expansion into new habitats, and maybe even led to changes in social interactions--for instance, if food could be more easily carried and brought back to share with infants and others (a behavior whose rudiments are seen in modern chimps).
In any case, it is an undisputed fact that bipedalism, once it emerged, rapidly became firmly established in the hominid line and that it continued to spread throughout a succession of later species. This suggests that this evolutionary "novelty" was, for whatever combination of reasons, fairly strongly "favored" by natural selection in a period which also seems to have corresponded with some significant and large-scale environmental changes.
Was the "second big leap" in hominid evolution--the slowing down of developmental rates and huge increase in brain size--also correlated with periods of major environmental changes? There is evidence to suggest that it was. The period around 2.5 million years ago (right around when the "second leap" took place) was a time when the global climate cooled down even further, when big ice sheets started covering the Arctic, and when large parts of Africa became much more arid (dried up). Where there had once been huge unbroken tropical forests, and then a patchwork mix of forests and woodland savannas, there now appeared much larger areas of open, dry and largely treeless grassland savannas. Again, periods of such dramatic environmental changes can easily lead to lots of species extinctions (and probably did!), but such changes can also create environmental conditions which favor the establishment and spread of significant evolutionary modifications and of whole new species. The newly extensive dry grassland savannas would have been a harsh environment for early hominids: sources of adequate plant foods could well have become more unreliable and more highly dispersed than in the traditional tropical forests and even the mixed woodland savannas of the earlier periods, and the relative absence of trees would have left the hominid groups vulnerable to big grassland predators, such as the large cats. Under such conditions, the emergence of any increase in tool-making and reasoning abilities and in social coordination would likely have been heavily favored by natural selection.
It may well turn out that the important environmental changes taking place in Africa some 2 million years ago (the drying out and the development of those more extensive grassland savannas) in effect indirectly "spurred" the further modification of the hominids in a more human direction. Again, one might think that natural selection would have eliminated any hominid lines that started to produce essentially "premature" babies that were totally helpless and could not fend for themselves for a long while; but the fact that such a change also happened to allow the hominid brain to develop for a much longer period of time after birth (allowing the new hominid infants to expand their mental capacities through social interaction and learning, rather than genetic programming, to an extent never before seen) probably more than compensated for any disadvantages.
Maybe all this could have happened even without such major environmental changes taking place. After all, an increase in a capacity to learn, to manipulate and refine tools, and to better communicate and reinforce aspects of socialization in an already social line of mammals could well be heavily selected for, even in a relatively unchanging environment. But the new challenges that were likely posed by the significant changes in climate, vegetation, available food sources, and exposure to predators in that general time period around 2 million years ago could certainly help explain why the new (once again quite "odd" for their time!) Homo species seem to have been fairly rapidly successful, and to have undergone yet another burst of expansion and additional species diversification over the next million years or so.
Interestingly, not all the hominid species of that period ended up evolving in the direction of modern humans. The "robust" line of Australopithecines , whose teeth and jaws suggest that they probably specialized in eating mainly tough plant foods in the arid savannas, show no evidence of significant brain expansion, and fairly soon became extinct. The "gracile" hominid lines, on the other hand--and especially the newly emerging species of Homo--seem to have maintained a more generalized diet (judging from their teeth and facial structures, which are typical of less specialized omnivores). It seems likely that they also started to consume increasing amounts of meat,which would have given them a wider (and likely highly nutritious) range of foods with which to sustain themselves in the increasingly harsh and dry environments. Even fossils of the "late" Australopithecine A. garhi (who lived in Eastern Africa just prior to the emergence of the first of the Homo species) have been found associated with antelope remains whose bones show stone tool "cut marks," a sign of having been butchered. Any increase in post-birth brain development in this period would likely have been very helpful for learning such new skills and would likely have been heavily favored by natural selection.
But it was the somewhat later species Homo ergaster who really took things one major step further, as this species seems likely to have been the first to figure out how to use and make fire . This was a tremendous innovation, because it allowed these hominids to travel out in the open and still keep predators away at night even when there were no trees to sleep in; and fire can also be used to soften up and render more digestible a variety of tough foods (such as fibrous roots and tough meats) through cooking .
Not surprisingly (what with their much bigger brains, increasingly refined stone tools, fire, and their likely more developed language and means of social coordination), Homo ergaster (also known as the African Homo erectus)was the species which seems to have been the first to venture out of Africa on a large scale and the first to succeed in establishing itself in a variety of different environments in many other parts of the world.
When our own species Homo sapiens emerged in Africa around 200,000 years ago (probably out of the African Homo ergaster/erectus or a very similar hominid species), it had even more developed cognitive (mental reasoning) abilities--as evidenced, for instance, by its stone tools, which were of significantly more complex design than those of Homo ergaster/erectus . Homo sapiens by this point likely had fairly developed abilities for recognizably human language, social interactions, and the general ability to transform itself and the world around it more through conscious cultural modifications than through any ongoing biological evolution (though even some of the later descendants of H. erectus , such as the Neanderthals, were capable of developing some significant aspects of human culture, with some of them beginning to bury their dead in ritual manner, to cite just one example).
One Species--All Over The World --A Species Radically Transforming the World
By the time our modern Homo sapiens species itself started to spread out of Africa around 50,000 years ago, it had the biologically based behavioral flexibility and the social coordination to range into just about any kind of physical environments and to adapt to them through cultural means (for instance, using animal skins and fire to ward off cold, improving on designs and materials in making tools used to harvest plant foods and hunt game animals for meat, etc). Homo sapiens now had the means to accumulate and transmit knowledge between groups--and across the generations--through all sorts of cultural means, including newly emerging art and ritual . Everywhere it went, it ended up replacing the leftover populations of the older human species who were the descendants of the earlier Out of Africa migrations undertaken by Homo erectus .
From our emergence in Africa around 200,000 years ago, we managed to fairly quickly spread to all corners of the globe, even making it to the Americas via the Bering Strait at least 12,000 years ago. We started off in Africa as one single species, and we have remained one single species ever since. (See Box "We All Came From Africa.") No pocket of modern Homo sapiens is ever truly reproductively isolated from the rest of the species, so we are continually intermixing our genes as we have done since our earliest origins on the African continent.
Most fundamentally, our species--which today builds computers and explores the depths of the oceans and the far reaches of space--is essentially unchanged biologically from the Homo sapiens who set out from Africa in that second migration wave some 50,000 years ago. This is not just because relatively little time has passed and individual species tend to remain relatively "stable" throughout much of their lifespans, but because the species that our richly diverse hominid ancestry ended up "spinning off" some 200,000 years ago had an unprecedented capacity to continually modify and restructure its own lifeways, and just about any aspect of its outside environment, through cultural means. This proved to be a much more rapid and effective way of changing the human condition than anything that could be accomplished through any ongoing biological evolution. The individuals responsible for the first prehistoric cave art, those who first ventured across the Bering Strait, those who maintained a hunter-gatherer mode of existence for 100,000 years or more, those who developed agriculture 10,000 years ago, and those who created advanced technological societies just in the last couple of centuries--were all basically the same people . (See Box "What Does the Science of Evolution Tell Us About Human Races"?) We have undergone no truly significant biological modifications (no further brain expansions, for instance) in all this time--though the changes we have effected in every aspect of the world around us through consciously initiated social and cultural modifications, in just a few tens of thousands of years, are astounding, to say the least.
We humans long ago evolved an unprecedented ability to continually learn new things, to consciously seek to modify and transform the material world, and to pass on a tremendous amount of accumulated information from one generation to the next through those non-genetic cultural means. This is what allows our species to "cope" with new problems and new opportunities presented by the outside world (or even to fail to do so!) without undergoing significant biological modifications of our bodies and without spinning off any new species. (See Box "Are Humans Still Evolving?")
This, however, doesn't mean we won't someday go extinct: every particular form of matter eventually ceases to exist as such, and humans--at least what we think of as being human today--will also eventually cease to exist. The real question may turn out to be whether the extinction of human beings is going to happen sooner rather than later, and what the quality of human life is going to be from here on out. Will we be able to use some of our tremendous capabilities to make repeated social and cultural modifications to avoid driving ourselves over the brink through war, social oppression, and global environmental degradation? The answer to that question will ultimately be up to us.
___________________________
Notes:
* This common ancestor species was most likely a "late" version of Homo ergaster/erectus , sometimes referred to as " archaic Homo sapiens. "
How
do we know this? A combination of reasons.
First, our DNA shows that our closest living relatives are the remaining species
of African apes (African chimps and gorillas), and not the Asian apes (the orangutans,
to which we are not very closely related). DNA "molecular clock" calculations
show that roughly 5 million years ago a species of African ape "split"
into divergent lines: on the one hand, the line which eventually produced modern
African chimps; and, on the other hand, the line which eventually produced modern
humans.
Second, it is only in Africa that the fossils of hominid species that
are more than 3 million years old have ever been found (and there are lots of
them). So there is general agreement among paleontologists and evolutionary
biologists that the whole process of human evolution started in Africa.
Third, the DNA evidence makes absolutely clear that the sole surviving species
of the human line (our modern Homo sapiens species) is one single
species the world over, and the oldest known fossils of our species—dated
at 100,000 years or more—have also been found in Africa. Fossils of anatomically
modern Homo sapiens don’t appear outside of Africa before 50,000 years
ago, the time when our species is thought to have begun to migrate out of that
continent.
Finally, while we know that our Homo sapiens species overlapped with
at least one or two other human species in parts of Asia, the Middle East and
Europe until as recently as about 35,000 years ago, the fossil evidence indicates
that these other species were anatomically and culturally more similar to the
Homo erectus species of 1 to 2 million years ago (from which they were
likely derived) than to modern Homo sapiens.
A question which is still sometimes posed is whether our single modern human
species definitely first evolved in Africa (and from there spread to all parts
of the world), or whether it might have evolved in some other part of the world—for
instance, as a spin-off of European or Asian populations of Homo erectus—and
then only later spread back into Africa and into other regions of the globe.
It has even been suggested that modern humans could have emerged out of some
kind of evolutionary convergence and "blending" of the two or three
pre-existing and probably closely related human species which had already spread
to different parts of the world by around a million years ago (such as Homo
ergaster/erectus in Africa, Homo erectus in Asia, Homo neanderthalensis
in the Middle East and Europe, etc.).
This so-called "multiregional hypothesis," which is sometimes still
put forward in the media, proposes that these populations could have eventually
come back into contact with each other and interbred to produce modern Homo
sapiens. But such suggestions don’t rest on a solid scientific basis. While
it is quite possible that there were, in fact, multiple waves of hominid migrations
out of Africa over the past million years or so (perhaps starting as early as
Homo habilis), and while there may even have been a certain amount of
back-and-forth migration (with some early migrant populations later returning
to Africa), there is no evidence to suggest that the modern species Homo
sapiens evolved out of any of these earlier migrant populations or out of
any kind of interbreeding and "blending" of their descendant populations.
Such an idea actually runs completely counter to what we generally understand
about how evolution works: biological populations which remain reproductively
isolated from each other for hundreds of thousands of years (as was likely the
case for those early migrant populations of Homo erectus) accumulate
significant genetic differences in that amount of time and are very unlikely
to be able to interbreed and reproduce even if they later come back into contact
with each other; and biological populations which have become geographically
and reproductively isolated from each other long enough to constitute separate
species do not, at some later point, then all start to converge on one single
evolutionary direction, resulting in them all coming together to form one single
new species. Evolution just doesn’t work that way. By contrast, what is very
commonplace in biological evolution is that a new species evolves in one single
location, out of a small and reproductively isolated off-shoot population of
a parental species, and then spreads out into new areas. The evidence which
strongly suggests that Homo sapiens evolved from a small population in
Africa and only later spread to all parts of the world fits this common pattern
of the origin of new species.
The techniques of modern molecular biology, applied to analyzing the DNA of
modern human populations, have provided additional evidence to support this.
Both the analysis of human nuclear DNA (contained in all cells and contributed
by both parents) and the analysis of human mitochondrial DNA (a form
of DNA present in cell organelles called mitochondria but which is passed from
generation to generation only through the mothers’ lines), along with some studies
of the distribution patterns of human genetic variation around the world, have
all converged on the same conclusion: our modern human species Homo
sapiens had a single African origin.
In fact, the mitochondrial DNA evidence strongly suggests that all currently
living human beings are descended from a single small population (though not
just one single woman, as is sometimes misrepresented in the press) which lived
in Africa around 150,000 years ago. Calculations derived from an analysis of
nuclear DNA came to the same conclusions. Our overall human genome obviously
contains DNA information that goes back even further in time, and some specific
human DNA combinations present in some of our ancestors have died out by now
(the same basic way human names can die out when some lines stop producing descendants).
But the molecular DNA calculations indicate not only that species like Neanderthals
did not contribute to the modern human gene pool (revealing, for instance, that
Neanderthals diverged from the hominid line that would later give rise to Homo
sapiens some 600,000 years ago), but also that all living humans still contain
segments of mitochondrial DNA that were present in just one single small population
of Homo sapiens that lived in one single geographical area 150,000 years
ago, long before Homo sapiens’ migration out of Africa (documented in
the fossil record) had even begun.
What Does the Science of Evolution Tell Us About Human "Races"?
The
main thing that evolution teaches us about race is that there is no such
thing as truly distinct biological races of human beings! What we generally
think of as human "races" are historically, socially, and culturally
defined categories; but these social categories do not actually correspond to
any real "natural" divisions of the human species.
To be clear: the social concept of "race" can (and does) still have
important social meaning for people’s lives—it can, for instance, be
an important form of cultural identification of both oppressed and oppressor
social groupings (whether in a positive social direction, as in something like
the Black Pride movement, or in a negative social direction, as in something
like the KKK or other white supremacists). Furthermore, the social concept of
race is still used to economically, socially, culturally, and politically oppress
and discriminate against entire groupings of people. So, clearly, the struggle
against racism and national oppression is far from over, and the struggle for
the implementation of "racial equality" and for the genuine emancipation
of oppressed peoples is something that still needs to be concretely fought
for. Any way you look at it, it is not yet the case that "race doesn’t
matter any more."
But all this has to do with the social reality of race. From a biological
standpoint, the concept of distinct races among human beings is essentially
meaningless.
This may come as a surprise to many readers, because we have all been so
socially conditioned to think that people belong to supposedly different
"races" based on average differences in such superficial features
as skin color or hair type. Many people already know that no one "race"
is in any way innately superior or inferior to any other "race"; and
many people may also already know that all human beings belong to a single
species. (We all pass the "single species test" because no human
population is ever completely reproductively isolated from the rest of humanity
and all human populations can, and do, interbreed and produce viable offspring
all throughout the globe).
But, despite all this, many people still mistakenly believe that so-called "races"
correspond to some kind of natural biological categories, and some people even
think that different human races are something like different breeds of dogs!
Nothing could be further from the truth. No matter how many different
ways you try to split and categorize humanity according to such superficial
differences as skin color or hair type (and regardless of whether you try to
split humanity into five or 500 different "racial types," depending
on how many features you choose to focus on), you will find that when you
get down to comparing human populations at the underlying molecular and genetic
level, all these socially defined and artificial "racial" categories
collapse! This is because the types and the amount of genetic variation which
exists both within and between local human populations never actually fully
correspond to any of the societal categories we usually define as the major
human "races"!
It’s not that there aren’t some patterns of genetic variation between
regional populations of human beings—it’s just that the observed patterns of
genetic variation tend to cut across all the so-called "racial" lines.
For instance, the genetic variation found in one population on one continent
might be more like that found in a population halfway around the world than
like that of a neighboring population, and it is a fact that there is not a
single gene (or single "allele," or alternative form of a gene) that
can serve to clearly distinguish one socially defined "race" from
another!
There are, of course, some well-known average differences, in populations from
different geographical regions of the world, in terms of the frequency of distribution
of some genetic alleles such as those conferring resistance to some diseases.
(For example, one of the hemoglobin alleles which can cause sickle cell disease
but protects against malaria is more common in people whose relatively recent
ancestors came from those parts of Africa and Asia where malaria is a common
problem; but even this feature is not always clearly correlated with what people
tend to think of as "race.")
Mainly, however, the majority of recognizable differences between broad groupings
of human beings have to do with very superficial features like skin color or
hair type: nobody would deny that, on average, it is usually pretty easy to
tell the difference between people of Bantu, Japanese, or Swedish descent, for
instance. But again, this is "on average," and there is not actually
any such thing as a "typical" member of any of the so-called major
human "races" anywhere in the world, even with respect to superficial
features like skin color and hair or body type.
For example, "Africans" encompass every imaginable skin color
and body type, including not only every imaginable shade of darker skin but
also Caucasian-looking North Africans and Egyptians or the yellow-skinned KoiSan
peoples in the South. And the African continent is home, at one and the same
time, to the (on average) shortest and to the (on average) tallest
human populations in the world (the Pygmies and the Masai).
Similarly, "Europeans" include not only many pale-skinned blond people,
such as the average Scandinavians, but also the often relatively dark-skinned,
dark-haired people of Southern and Eastern Europe, who often look a lot more
like some of the average populations of North Africa and the Middle East than
like the average Swede. "Asians" also don’t fit a single stereotype:
they include a wide range of people including Turks, Indians, and Japanese,
and overall they encompass every imaginable skin color and body type.
As for "Hispanics" or "Latin Americans," they include people
of Southern Chilean ancestry who look more like pale-skinned Canadians than
like their brown-skinned Amerindian neighbors of the tropical zones of Latin
America, as well as many Blacks from the East coast of Brazil or Central America.
So everywhere you look, you will find a wide variety of skin colors and body
types (some of which can be traced to the "mixing" effects of more
recent migrations and invasions, but some of which seem to go back further in
time and, like the higher frequency of darker skin in the tropical zones and
the higher frequency of pale skin in the temperate and Arctic zones, may have
possibly originated as local adaptations to certain local environmental conditions
in the early history of human global expansion—see below). But today the overall
variety is so great that it really doesn’t make any sense to talk about a typical
"African," a typical "European," a typical "Asian"
or a typical "Hispanic," even in terms of those most superficial of
features such as skin color.
Even more importantly, if you look even deeper, at the underlying molecular
and genetic variation present in the DNA of every single local human population,
you will find that, there is always more overall genetic variation between
individuals in a given local population than there is between any two geographic
populations, or between any two socially defined "racial" groups,
anywhere on earth. In fact, there is now a widespread consensus among anthropologists,
molecular biologists, and population geneticists that the concept of biological
"races" in human beings is in the main meaningless, since pretty much
the entire amount of genetic variation present in the entire human species
can be found inside any one local human population anywhere in the world.
For example, as the population geneticist Richard Lewontin often points out,
if all humans on earth became extinct except for just the Kikuyu tribe of East
Africa, this one tribe would be enough to preserve at least 85% of all
the genetic variability contained in the human species as a whole.
A genuine biological race (also called a geographical race or a subspecies)
is defined as a population of genetically variable individuals which interbreed
among themselves but which consistently maintain a relative proportion (or "relative
frequency" ) of certain specific gene forms (alleles) which is different
from that found in populations of the same species living in a different geographical
area.
Different local human populations do sometimes differ in the relative frequency
of certain genetic alleles (of those genes that come in multiple alleles). For
instance, some local human populations on average have a generally higher or
lower frequency of the A, B, AB, or O blood types; some have a higher frequency
than others of the sickle cell hemoglobin allele which protects against malaria;
some have a higher than average frequency of an allele which facilitates milk
digestion than most human populations which are generally lactose intolerant
as adults. But these kinds of occasional regional populational differences in
the proportion of specific alleles never really neatly or consistently correspond
with the so-called major racial categories. For instance, African Americans,
whose historical ancestors were primarily West Africans from regions of Africa
where malaria is prevalent, on average do have a higher frequency of the particular
hemoglobin allele which can cause sickle cell disease (but also protects against
malaria) than Caucasian Americans whose mainly European ancestors lived in areas
that didn’t have malaria. But, on average, their frequency for this allele is
lower than in African populations which are currently still frequently exposed
to malaria. And, even more importantly, if you also look at different regional
populations in Africa itself, you will find that among different populations
of sub-Saharan "Blacks," there are populations from highland mountainous
areas (where malaria-bearing mosquitoes cannot live and malaria is not a problem)
who tend to have a much lower frequency of the sickle cell allele that protects
against malaria than either African-American Blacks or populations of Black
Africans of the tropical lowland areas where malaria is a big problem. And yet
most people would tend to place all these people in the very same
"racial category."
Again, from a biological standpoint there is no such thing as a typical "African,"
a typical "Black African," a typical "African American,"
or a typical "Black," just like there are no typical "Whites,"
no typical "Asians," and no typical "Hispanics." And the
same can be said about every single other racial category people have ever tried
to socially define.
The reason there are no true biological races of human beings is quite simple.
The one and only current human species (Homo sapiens) must have started
off (like all new species) as an initially small offshoot population evolving
out of a preceding parent species (probably Homo ergaster, the African
version of Homo erectus, or some very similar hominid species). But we
also know from the fossil record that our species, Homo sapiens, which
evolved roughly 200,000 years ago, was able to greatly spread and expand into
a wide variety of habitats beginning at least 50,000 years ago. So, within a
relatively short period of time (in evolutionary terms), we had spread out of
Africa and into every conceivable habitat and climactic zone in the Middle East,
Europe, Asia, Australia, and even crossing over from northern Asia to the Americas
by at least 12,000 years ago.
The scientific evidence suggests that our species hasn’t undergone any truly
significant biological modifications in the past 100,000 years. What has
changed a lot has been human culture, our capacity to develop, transmit, and
continually build upon ever-increasing stores of human knowledge and experience
passed on from generation to generation through non-genetic cultural means,
thereby becoming the first species on earth able to change itself and its
surroundings (rapidly and dramatically) primarily through non-biological cultural
means, bypassing and outstripping the much slower and more constrained (limited)
mechanism of biological evolution.
Of much greater overall significance than any of the genetic variation among
individuals is the fact that the very biological features which made us distinct
in the first place—the combination of fully bipedal locomotion (which more consistently
freed our hands) along with the significant post-birth period of brain development
(which made possible unprecedented amounts of socialized teaching and learning
and accompanying leaps in social coordination and communication)—gave us an
unprecedented ability to adapt to every imaginable kind of earth environment
through cultural adaptations and modifications rather than through the
more constrained and slow-going processes of biological evolution.
Furthermore, it is only early in the history of our species (and only temporarily)
that local populations could have remained somewhat cut off from each other
for any length of time. Some of the small differences in things like average
skin color of populations with different regional ancestry may reflect some
degree of biological adaptation to local conditions in that early history. For
instance, all around the globe, modern populations whose ancestors lived in
tropical zones (which get a lot of UV radiation from sunlight) tend to have
darker skin (more melanin pigment) than populations whose ancestors lived in
temperate zones closer to the poles (which get much less sunlight and UV radiation).
It has been suggested that this may have been the result of adaptations of local
populations to local conditions, because darker skin protects against UV-caused
destruction of folic acid (an important nutrient in the reproductive years,
since it prevents birth defects such as spina bifida), whereas lighter skin
makes it easier to produce vitamin D (which is important for calcium metabolism
and the development of healthy skeletons in areas with insufficient sunlight).
It is therefore possible (though not absolutely certain) that average skin color
differences in local geographic human populations emerged early in our history
due to reproductive advantages of different skin colors in regions having different
total amounts of sunlight. (For more on this proposal see, for instance, the
article by Jablonski and Chapman in the Oct. 2002 Scientific American.)
But it remains the case that by far the greatest amount of genetic variation
present in the human species can be found in the variation that exists among
individuals within any particular human population. In the relatively
short amount of evolutionary time since modern Homo sapiens first appeared
(about 200,000 years ago), no human population has ever remained completely
reproductively isolated (cut off) from other human populations for anything
like the length of time (the great number of reproductive generations) that
would be required for sufficiently significant genetic differences to begin
to accumulate between the populations and turn them into truly distinct geographical
races.
Some populations differ in relative frequencies of genes that occur in variable
forms (such as those coding for blood types), but you cannot predict "race"
from these patterns of variation. As Richard Lewontin again points out, "The
Kikuyu of East Africa differ from the Japanese in gene frequency but they also
differ from their neighbors the Masai...the social and historical
definitions which put the two East African tribes in the same ‘race’ but put
the Japanese in a different ‘race’ were biologically arbitrary."
This is why even in medicine (where it may actually be worth reflecting on an
individual’s apparent ethnic heritage in order to not "miss"
certain historical differences in susceptibility to diseases like sickle cell,
and so on), the subjective appearance of "race" is still not as valuable
as an individual profile and can in fact be very misleading. In any case, individual
family and personal history and an analysis of social factors disproportionately
affecting the health of certain socially defined groupings (such as the
many effects of poverty on the health of ghetto dwellers, or the prevalence
of eating disorders in affluent adolescents) is a much better predictor of what
medical care is likely to be needed than any subjective evaluation of what racial
category a person appears to "fit."
It is important to remember that, throughout the history of our species, human
groups have continually migrated in and out of different zones, continually
interbreeding and ensuring an uninterrupted gene flow between populations which
eventually stretched across the entire globe. Small- and large-scale human migrations
have characterized our entire history as a species and continue to this day,
ensuring both our continuing biological unity and ongoing cross-cultural exchange
and enrichment.
The disgusting and ignorant attempts to try to preserve the supposed "racial
purity" of one "race" vis-à-vis others (such as promoted by Nazis,
Aryan Nations, KKK, and other such racial supremacists) is, among other things,
a genuine absurdity that doesn’t have any kind of scientific leg to stand
on! Leaving aside the fact that biology can clearly prove that there is no such
thing as innately "superior" or "inferior" peoples or "races,"
many of us recognize—and celebrate—the fact that what the supremacists think
of as their worst nightmare (so-called "race mixing") has essentially
already come to pass! We are and we have always been one single—variegated
but biologically indivisible—worldwide species.
[Readers interested in learning more about why the concept of biological race
doesn’t apply to human beings and related topics might want to especially check
out the recent book The Emperor’s New Clothes: Biological Theories of Race
at the Millennium by Joseph L. Graves, as well as relevant sections in Mismeasure
of Man by Stephen Jay Gould and Not In Our Genes by Lewontin, Rose
and Kamin.]
To answer this question it is important to keep in mind the difference between
the mechanisms of biological evolution and the mechanisms of cultural evolution.
The human species, like all other species of plants and animals, is made up
of populations of variable individuals. Part of that variation is genetic and
therefore inheritable and potentially subject to natural selection. For instance,
if you carry the gene for cystic fibrosis in your DNA, there is a chance you
might pass it on to your children.
However, a great deal of human individual variation is cultural—it is a product
of human learning and social experience—and these things cannot be transmitted
to descendants through biological reproduction. This is the case with things
like personality or social status, for instance. Wealthy people might "pass
on" wealth and privilege to their children (giving them money, education,
social advantages, etc.), but this has nothing to do with any of the genes passed
on through their eggs and sperm. Or, if people say your kid has "inherited"
his uncle’s short temper or your sense of humor, it is quite possible that your
kid learned these things by example or imitation; but this has nothing
to do with biological inheritance and the genes you passed on.
Genes make proteins—proteins that are important for the functioning of many
parts of our bodies. But things like personality, intelligence, social status,
etc., come about as a result of the many complex social experiences and interactions
between individuals and the outside world, and such things are simply not encoded
in the genes. Compared to all other species on earth, including all the other
primates and the many different hominid species who were our most recent ancestors,
the thing that makes us the most different is our much greater ability to learn,
to teach, to make things that never before existed, to invent new ways of interacting
with each other and the environment, to organize and to communicate, to transform
ourselves and the natural and social world about us—all without having to
undergo any biological modifications.
Our close relatives, such as the chimpanzees, can do many of the same things
to a certain degree. (They have complex forms of social cooperation and communication,
use simple tools, teach their young complex skills and form "friendships";
and in different geographic areas chimpanzee populations even develop somewhat
different "cultural traditions" in things like tool use or social
behaviors.) But none of this comes close to what human beings are capable of.
Our modern human species is the first species in the history of biological evolution
on this planet to break out of the constraints of biological evolution and "evolve"
primarily through non-genetic social and cultural means. This is, in very large
part, what makes us human.
In fact, non-biological cultural evolution has become so much more important
than biological evolution in effecting changes in people that how well people
survive, and how many offspring they leave generally has little or nothing to
do with any advantageous or disadvantageous features that genetic variation
and genetic mechanisms might bring about. Whether a disease kills you or not
has much more to do with whether you get a vaccine, or antibiotic, or other
medical treatment—or perhaps just clean water and fresh food—than it has to
do with any genetic variation you might have relative to other individuals.
How many descendants you are likely to leave in subsequent generations has much
more to do with social factors (relative poverty or wealth and access to resources;
customs, traditions, views, and practices regarding birth control and the status
of women; the economic structure and organization of society, favoring large
or small families; influences of religion and other ideological factors, and
so on) than it has to do with features that you can pass on through genetic
material and genetic processes.
In the past 100,000 years or so, our bodies (including our brains) seem hardly
to have changed. And it is with the same basic biological bodies that
we went from a stone tool culture to being able to cure many diseases and to
explore (through the use of technology) remote parts of the cosmos. Again, all
this has been accomplished primarily through cultural, not biological, evolution,
though it is our very biology which made this possible in the first place.
So, has biological evolution come to a complete standstill in the human
species? Well, not quite, but almost. Human populations are still made up of
genetically variable individuals (we’re not clones of each other), and the relative
frequency of certain alleles (gene forms) in any given locality can still be
affected from one generation to the next by the continual genetic "reshufflings"
accomplished through sexual reproduction, or even by such things as occasional
genetic mutations or shifts in gene frequencies due to random factors like deaths
of individuals or migrations of individuals in or out of an area.*
As in all other species, if human individuals inherit some genetic variation which provides them with some features leading to some kind of reproductive advantage (allowing
them to produce more children, who can themselves successfully reproduce, than
individuals who don’t have these new inheritable features) and if this
process gets repeated over a number of successive generations, it is still possible
for some small-scale evolutionary changes (with regard to disease resistance
for instance) to become manifest in local human populations.
But in practice this kind of thing is very rare, because most of the changes
still occurring through genetic reshuffling in human beings don’t significantly
affect how many descendants individuals end up contributing to future generations.
As we have seen, in modern times, how many descendants a human individual manages
to contribute to succeeding generations has very little to do with any kind
of biological "reproductive fitness" and much more to do with social
and cultural relations and customs, and relative opportunity—things like whether
or not people have enough to eat, views regarding how and when and with whom
people should or should not reproduce, our ability to prevent and cure many
diseases that would have previously prevented reproduction, etc. All these
kinds of things today have much more effect on human reproduction than any new
features that might be produced through random genetic reshuffling of our DNA.
In fact, even going back into the distant past of human existence, the ability
of human beings to transform ourselves and our world through cultural means
has, for a long time now, so far outstripped the effects of any biological evolution
that there is no evidence that our bodies have undergone any truly significant
biological reorganizations through selection proceeding in any well-defined
direction in the past 100,000 years!
On a small scale, it may still be possible to find recent or ongoing evidence
of human populations evolving such things as differential resistance to disease.
Many current human diseases (including many cancers) are not subjected to natural
selection simply because these diseases don’t affect reproduction, or because
they affect people mainly in later years, after they’ve already had children.
On the other hand, it was only a few hundred years ago that European colonizers
devastated populations in North and South America by introducing smallpox (sometimes
on purpose!) to Amerindian populations which had never before been exposed to
this disease of Europeans. The Europeans, whose populations had been previously
exposed to smallpox for hundreds of years, had evolved partial immunity to this
disease over many generations. Because of this they generally got much milder
cases of smallpox and often survived. By contrast, the Amerindian populations,
which had never before been exposed to smallpox and had therefore not had time
to evolve any degree of immunity, caught the disease full-force and were nearly
wiped out by it (a factor which contributed significantly to their military
defeat at the hands of the Europeans).
It is not difficult to imagine that today any chance mutation which might provide
resistance to something like HIV (which kills many people as youth and before
they’ve been able to have children) could take hold and spread over generations
in Africa and other parts of the world being particularly devastated by this
disease. One such mutation seems to be present already in a tiny percentage
of European Caucasians and is thought to have become established through natural
selection a few centuries ago, during the bubonic plague epidemics of the Middle
Ages, where it may originally have protected against that disease. Cases such
as this are interesting and leave open the question that our species may still
be evolving in some limited ways in relation to things like lethal diseases
which strike people prior to reproduction.
But even with such devastating diseases, it is evident that what we could accomplish
today through social and cultural transformations is much greater—and much faster—than
anything that is likely to be accomplished by relatively weak natural selection
in this day and age. The reason millions of people are rapidly dying of AIDS
at a young age in many African nations—while at least some people are able to
"live with" HIV for much longer periods of time in the industrialized
nations—has much more to do with the effects of poverty and the lopsided relations
and exploitation of the imperialist system worldwide, with the denial of education
and especially the unconscionable withholding of necessary modern medications
and treatments by the profit-oriented multinational pharmaceutical companies,
than with anything having to do with biological evolution. And the solution
to such problems doesn’t have anything to do with biological evolution either!
Our own species evolved out of pre-existing hominid species—so will we ever
spin off an entirely new species?
This is not likely. The reason for this has to do, first of all, with everything
that has just been said about how cultural and social means of modifying ourselves
and our external world have far outstripped anything that could be accomplished
through natural biological evolution (though what we may end up doing to the
biology of our species through things like genetic engineering remains an open
question).
In addition, it is important to remember that a brand new species generally
evolves out of a small, reproductively isolated offshoot population of
a parent species. Without that period of absolute reproductive isolation, spanning
a significant number of reproductive generations, it is not possible for significant
genetic modifications to take hold and sufficiently distinguish a new population
from its ancestral population and make it impossible for them to once again
interbreed and merge back into a single species. But such reproductive isolation
just isn’t going to happen with human beings on this planet—we are at this point
one single, highly mobile and globally distributed species. We inhabit
every single corner of the earth and every single kind of habitat, and there
is continuous gene flow among human populations. It is simply not possible
at this point for any human population on this planet to become completely reproductively
cut off from the rest of humanity for the great length of time that would be
required for that population to even begin to diverge as a new species
(even if all those cultural factors didn’t subvert the process anyway).
I suppose that if a small population of human beings ever colonizes a distant
part of the cosmos, and could somehow remain there completely cut off from earth’s
humanity for a great many generations (how likely is that?), then a certain
amount of biological evolutionary divergence could conceivably take place. Even
then, however, particularities of individual genetic inheritance would be highly
unlikely to play the principal role in determining the make-up of successive
generations. Whether the divergence would be sufficient to permanently prevent
future successful interbreeding of the space colonists with the ancestral earth
population (the biological definition of full speciation) is still likely to
be primarily dependent on non-genetic cultural and social divergences, including
how future societies approach questions like genetic engineering and whether
or not they even reproduce biologically any more in the ways that are familiar
to us.
The mechanisms of biological evolution produced every living species on earth
over some 3-1/2 billion years, and there is absolutely no doubt that our own
species was produced by this natural process. Like every other living species,
our own species will one day go extinct, in one way or another. But between
now and then, we will no doubt consciously transform ourselves and the world
around us in ways which we can barely yet imagine.
__________________________
Footnotes:
* However, these
kinds of random factors, which can have big effects on the overall genetic make-up
of especially small and isolated populations in other species, tend to
have negligible effects on the sorting out of genes in human beings, simply
because human populations are never completely reproductively isolated from
each other and there is in effect continual gene flow among them.
This article is posted in English and Spanish on Revolutionary Worker Online
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