The Evolution of Human Beings

The Science of Evolution

Part 6a: Where Did We Come From?

by Ardea Skybreak

Revolutionary Worker #1179, December 15, 2002, posted at http://rwor.org

Who are we? Where did we come from? What might the future be like?

These are questions human beings have probably been asking themselves for as long as there have been human beings. It's one of the things that makes us human--this ability to think and wonder and converse with each other not just about present-day events, but also about what happened in the past, or about what might happen in the future. No other living species on this planet can do that. It is this very ability which allows us to learn from the accumulated knowledge and traditions of our ancestors (some of which we may want to tap into and some of which we would no doubt do better to discard, but all of which we can learn from in any case); and it is this ability which allows generation after generation of human beings to keep building onto the accumulating storehouse of knowledge and experience, both by revising and modifying past understanding, and by gaining additional new insights in light of our ongoing explorations and transformations of the world around us.

But for all this inspiring ability, we humans also seem to have the ability and the tendency to just "make stuff up" when we simply don't yet know the truth about something! When we make up imaginative stories about the future, we usually call it science fiction , whereas imaginative stories about the past are generally referred to as superstitions or myths . As we pointed out in the first installment of this series ("The Science of Evolution: Part 1, An Overview" [ RW #1157, June 30, 2002]), a particular set of myths known as "origin myths" or "creation myths" can be found at the core of the many religions that are still practiced around the world today. (And such myths were no doubt at the core of all the many religions of the ancient world which have by now ceased to exist.)

Creation myths are simply the many imaginative stories which people who lived hundreds or thousands of years ago told each other in an attempt to explain what they did not yet have a basis to understand: how people first came to be . These stories (told through songs and poems in many cultures, and written down in human-authored books of "holy scriptures" of various sorts, such as the Bible, the Tanak, or the Koran) differ from each other in their details, and often reflect the fact that the people who originally told these stories lived in a variety of different times, places and environments. But all the creation myths also have a great deal in common: whether they speak of such things as sun- gods, water spirits, earth mothers, or bearded patriarchs in the clouds. All the creation myths try to explain "where the first people came from" through an imaginative story about how some time, long ago, the first people were created by some mysterious (and usually invisible) supernatural spirits, which were often said to have molded human beings out of some earthly elements (such as clay), and then either lowered them from the skies or guided out them out of the bowels of the earth, or some such thing. And from then on, the people were able to go forth and populate the earth (or at least whatever small part of the planet the authors of these myths happened to be familiar with at the time).

I don't find it at all surprising that ancient people, living in a pre-scientific world , made up such stories to attempt to explain human origins. And while studying these stories today won't provide us with a real explanation about where human beings came from, a number of them can still be appreciated as poetry, song, and literature, and all of them can still teach us a lot about how various ancient peoples lived, and how they thought about their world.¹

But while I find it easy to understand why people long ago made up creation myths, the fact is that there is more than enough historical and scientific evidence to demonstrate that all the different creation stories of all the world's religions were themselves "created"-- by human beings ! Human beings also create stories for children about such things as Santa Claus or the tooth fairy--but in those cases we all know that we're supposed to grow up and recognize them for what they are: stories with a social purpose perhaps, but stories nonetheless.

The reasons human beings, even as grownups, seem to cling to religious creation stories about the origins of human beings and other life-forms has a lot to do with the social reasons many people feel they need religion in the first place. But it also has to do with: a) the simple fact that, for most of the time human beings have existed, we did not yet have the scientific methods and scientific outlooks (and hadn't yet made the kind of scientific discoveries) which could give us real answers about where people came from, based on fact, rather than on superstition and myth; and b) a genuine scientific knowledge and understanding of things has traditionally been denied to the majority of humanity as a direct result of the social divisions and inequalities that exist throughout the world--a situation which continues to this day.

Today science can clearly and definitively answer the basic question: "Where did we come from?" Ever since Charles Darwin's groundbreaking work on the evolution of life was published in the late 19th century, scientists have increasingly been able to understand and scientifically demonstrate how all the different forms of life on this planet (all the plants and animals, including people) are related to each other by varying degrees, and the fact that all the different species evolved (originated and changed) over hundreds of millions of years, through a process known as"descent with modification ," out of a series of common (shared) ancestors . And it is now very clear that this process was shaped in large part by that prime mechanism of evolutionary change known as natural selection .²

Evolution by natural selection operating on populations of varied individuals was Darwin's "great idea"; but the fact that evolution occurs in this way is no longer just an "interesting idea" or "unproved theory" or speculation--it is a well-tested scientific fact . It can even be said that pretty much the entire progress made in biology and related sciences between the beginning and the end of the 20th century (and continuing today) has constituted one long "proof" of Darwin's basic theory of evolution. By now it has been demonstrated, well beyond the shadow of a doubt, that life has in fact continuously evolved over the past 3-1/2 billion years, that life is continually evolving, and that much of both small-scale and large-scale evolutionary change takes place through the unconscious mechanism known as natural selection. Modern science has not only proved that evolution occurs but has demonstrated how it occurs, including through discoveries in the science of genetics and molecular biology, which did not yet even exist in Darwin's time.

The evidence of past evolution is all around us, in every living species and in every fossil of long dead species. And there are literally thousands of scientific studies which have shown us that evolution is still going on all around us. Populations of living plants and animals are always changing (evolving) over many successive generations (not "instantly") thanks to natural selection and related phenomena. (For a fuller explanation of how all this works and some examples of evolution in action in the world around us, see the previous installments of this series).³

Human Beings Evolved out of Preexisting Nonhuman Species

Is there actual evidence that human beings (and not just the other species of life that inhabit the earth) are the products of such biological evolution? Is there really clear proof that we are descended from some previous nonhuman species?Yes. Absolutely, without a doubt. The evidence of this is very clear.

Human beings are actually descended from a long series of preexisting nonhuman species. The evolutionary line which led to modern human beings (known as the "hominid" line) actually diverged (split off) from a line of African apes just a few million years ago. Another branch of this line split off to eventually give rise to modern gorillas and chimpanzees, our closest evolutionary cousins.

The "hominids" on our side of this evolutionary "split" include all the species that are considered to be more closely related to humans than to chimpanzees. As we will see, there have actually been many different species on "our" hominid side of the branching event, which were all distinguished from their ape ancestors by being bipedal (upright-walking). They represent various intermediate evolutionary steps linking the oldest ape ancestors to modern humans. The oldest of these upright-walking (bipedal) species were still very ape-like in many ways, while the relatively more recent ones include some which were much more like modern humans.

Paleontologists (scientists who collect and analyze fossils) keep finding more and more fossils of the many different species of bipedal hominids (some of which coexisted in the same time period), and much is being learned about which of these species were our most direct ancestors and which ones represent side- branches on our family tree which ultimately led to evolutionary dead-ends. Today's human beings all belong to one single species, the only one remaining out of the series of diverse hominid species. In the process of learning about these past ancestor species and the environments in which they lived, we are learning more not only about who our most direct ancestors were, but also about what it means to become fully human.

With each new fossil find and subsequent analysis and discussion by various teams of scientists, we keep learning more and more details about exactly how human evolution unfolded. There is still much to be learned, but the basic story, the basic pattern, is so clear and so well documented that the vast majority of scientists the world over would look at somebody who didn't believe human beings evolved from a preexisting species the same way they would look at someone who still believed that the earth is flat and that if you sail a boat to the horizon you'll fall off the edge!

The religious fundamentalist Creationists prey on the ignorance and confusion that are created by a lack of education and by mis -education: They literally lie and distort the facts that are known about evolution, and they even try to make people feel ashamed of their prehuman roots and ancestry. Creationists make it sound like evolutionists are telling people that human beings are "no different than apes in a zoo." But that is not the case. The truth is that evolutionists tell people that there is plenty of concrete proof that: 1) humans are in fact descended from ape-like ancestors; 2) modern apes like chimpanzees are very closely related to humans; 3) humans and apes still share many physical and behavioral features; 4) human beings also have some unique evolutionary features all their own, which obviously result in humans being quite different in many ways from their closest living ape relatives.

Creationists also like to claim that if schools and textbooks teach our kids that they are descended from animals, then the kids will just "act like animals" and sink into wanton immorality. This too is a ridiculous assertion, which profoundly disrespects youth, their families, and their ability to handle the truth. And it also once again fails to recognize that the fact that human beings evolved out of earlier nonhuman species doesn't mean that we don't have some special and uniquely human features which distinguish us from even our closest relatives among other species. The ways we act (for better or for worse) are--and can only be--distinctly human.

The science of evolution reveals that if you could walk back through time, through the entire series of ancestor species which make up our heritage, you would find not only the long series of prehuman ape- like species which make up our most direct ancestors, but way back before them you would eventually get back to the very first mammals (from which all later mammals, including such different species as bears, whales, dogs and people were all eventually derived, though at different points in evolutionary history). Those first mammal ancestors of all later mammals were rat-sized creatures that lived at the time of the dinosaurs. They themselves had evolved from a branch of mammal-like reptiles, and the first representatives of the reptiles in turn had evolved from a branch of some of the first amphibians (the group to which salamanders, frogs and toads belong) which were the very first creatures to crawl out of the water and walk on land. These in turn had evolved out of a particular branch of the marine fishes (whose bony fins and air bladders had formed a basis for the stumpy legs and primitive lungs of the first species to crawl out onto land and breathe there). As for the fish, they had evolved quite a bit earlier from a branch of marine invertebrates (animals without backbones), which had in turn evolved from some even simpler and more primitive marine creatures, going back all the way to some of the very first bacteria-like forms of life which started the whole evolutionary show going, and which would have looked like little more than a few strands of DNA surrounded by some simple cell membrane.

So, if you really want to talk about human roots and ancestry and get the full picture, don't just talk about our ape ancestors--take it all the way back! Take it all the way back to those first bacteria-like creatures which emerged out of the chemical soup of the world's early waters some 3-1/2 billion years ago and recognize them as your ancestors too!

I don't know about you, but I don't find this history frightening or disturbing in any way. Personally, I think it's actually pretty wild and wonderful that we got to be what we are today thanks to such a diverse mix of inter-related ancestor species. But recognizing the scientific evidence for all this doesn't mean that we're all going to start acting like bacteria, fish, or apes! We can only "be," and act like, what we actually are--a distinct and rather unique species, Homo sapiens.

But, some might ask, if we are in many ways distinct and unique as a species, then why bother trying to find out about the previous species we evolved out of? Are we just trying to put together some kind of evolutionary photo album of long gone ancestors? What is the point? Well, some people might want to study our origins and immediate ancestors just out of general interest and curiosity. (And there is certainly nothing wrong with that!) But I would say that, beyond that, it is important to try to learn as much as possible about the pre-existing species from which the human species was derived for two basic reasons:

The more we learn about the concrete reality of human evolutionary origins, the more people are likely to free themselves from all sorts of superstitions and belief in the supernatural--beliefs which I would argue are not just out of date but actively harmful to human beings. Many people who believe we were created by some kind of supernatural spirit also tend to rationalize all sorts of social ills and injustices, which they often explain away by saying, "it's just god's will" or "god works in mysterious ways." And many people passively wait for that same spirit to come to humanity's rescue , instead of taking some initiative to do something about social ills and injustices themselves. But if we understand the actual facts of modern science, and that human beings, while special and unique in many ways, are nevertheless simply the result of a very long and completely unconscious process of biological evolution and natural selection that has been operating on all living things for billions of years, then I suspect this will help many of us put things in proper perspective and that we will spend a lot less time looking to the skies and some supernatural spirits for salvation and instead look more to ourselves--to each other--to figure out what needs to be done .

The more we understand our own evolutionary origins, the more I think we will understand at one and the same time that our own species is closely intertwined, interconnected and interdependent with many other forms of life on this planet, and that the human species therefore can't (and doesn't) "go it alone." One important implication of this is that it can help us understand that if we keep wantonly degrading entire ecosystems and driving so many other species into extinction, we may end up unleashing a cascade of biological effects that could make the earth uninhabitable even for our species. At the same time, if we understand better what occurred biologically when the human line evolved out of its pre- existing ancestor species--what were the key evolutionary modifications and innovations involved with this--then I think we will also better understand such things as the fact that the evolution of our unique degree of behavioral flexibility and unprecedented capacity for learning new things actually frees us to a great extent from being rigidly determined by our underlying genetic programs. (We are so much more than our genes!)

We humans have a tremendous ability to dramatically transform all sorts of aspects of both nature and society, for better or for worse. This ability flows directly from some aspects of our biological evolution, which produced a species with a highly developed capacity to consciously interact with the world around it on the basis of continual learning, assisted by a highly developed capacity for social communication and social coordination.

We are not the only living species to have developed a capacity for learning, some significant behavioral flexibility, an ability to manipulate the external environment, and some complex forms of social organization and communication. These abilities can also be found to varying degrees in other species, especially among the other social mammals. To cite just a few examples, chimpanzees are able to make and use some primitive tools, dogs figure out how to "beg" for attention, wolves teach their pups how to hunt, elephants learn by example how to take care of their young, whales teach their offspring the complex "songs" their species use to communicate, etc., etc. So, especially among the social mammals that live in groups, there are many species besides human beings that have evolved some impressive abilities to learn complex skills that are not simply genetically programmed.

But it would be difficult to deny that the evolution of human beings represented a real qualitative leap in the development of such abilities. No other species on this planet comes anywhere close to human beings when it comes to the ability to actively and consciously transform the world around us. We should better understand the nature of that leap and its evolutionary origins precisely because it is what makes us so distinctly human.

Even in the late 19th century, Darwin and other scientists had noted the obvious physical similarities between humans and modern African apes (gorillas and chimpanzees) and already suspected that human beings must have evolved as a separate branch out of some kind of distant shared ape ancestor. Darwin and his friends turned out to be right, though absolute proof of this did not come until the 20th century.

Ever since Darwin's time, the Christian religious fundamentalists have done everything they could to try to keep people from hearing about scientific theories of evolution and about the vast amounts of concrete scientific evidence which have come to confirm the truth of these theories. They just don't want people to find out the proven facts about how life on earth goes back 3-1/2 billion years, because a literal interpretation of the Bible suggests that the earth is only a few thousand years old. The Creationists don't want people to learn about the scientific evidence which proves all the plant and animal species are related to each other to varying degrees, and that each and every species is the result of the evolutionary modification of a pre-existing species, because the Bible says that God created all the plants and animals separately and all at once. And, above all, the Creationists don't want to hear anything about how humans evolved out of a pre-existing nonhuman species, because the Bible says humans are god's "special creation," that we were created "in god's image," and that we were specially designed to lord it over ("have dominion over") all the other species on earth. The facts of evolution run counter to all of this. (See the box "Humans and Dinosaurs?! Another Creationist Absurdity.")

Of course not all religious people are fundamentalist nut-cases! Today, quite a few more open- minded Christians and other religious people accept evolution as scientifically proven fact, and choose to adapt their religious beliefs to take into account the overwhelming scientific evidence for evolution. Many feel, for instance, that the human beings who wrote the various books of the Bible many centuries ago necessarily had limited knowledge, and that what is said in the Bible should therefore not be taken too "literally."

The Catholic Church had to make a similar "adjustment" when it finally had to admit that it had been wrong to oppose the Copernican theory as heresy and blasphemy. But, for several centuries, the Copernican theory--which showed that the earth was not the center of the universe and that the earth and other planets revolve around the sun, and not the other way around--was vigorously and viciously opposed and even suppressed by religious authorities because it challenged the Biblical view that human beings are at the center of god's special creation. But ultimately there was no denying the concrete scientific evidence: Copernicus was right! ⁴

Both the Copernican theory and Darwin's theory of evolution shook up the world of organized religion and were strenuously opposed and denounced as heresy by religious authorities because both knocked human beings off our high horse (so to speak) and revealed that human beings and the earth were not some kind of special center of everything, as put forward in the Bible. Today's continuing denunciation of Darwinian evolution by religious fundamentalist Creationists (even in the face of overwhelming scientific evidence) is very reminiscent of the irrational rantings and ravings of the Church against Copernicus and Galileo hundreds of years ago.

As a side point, isn't it chilling to realize that the current President of the United States--the most powerful country on earth and one which admits to having large stockpiles of weapons of mass destruction- -is a self-described born-again fundamentalist Christian, who openly and unabashedly shares and vigorously promotes many aspects of the political and social agenda of the Christian Fascist creationists in the so-called "Religious Right"?

Learning about evolution (including human evolution) first and foremost is learning about truth- about proven scientific facts. But learning these facts and learning to unmask the lies of the Creationists will hopefully also help to strike a blow of resistance against a reactionary political and social agenda.

So what are some of the main things to know about human evolution? Well, first of all you should know that there is now absolutely zero doubt that we are indeed closely related to the modern-day African apes, which include the gorillas and the chimpanzees. The point of highlighting our relationship to the living African apes is that they are by far our closest living relatives. Therefore, studying modern-day living apes can help us to better understand the many aspects apes and humans still have in common, which are also likely to have been features possessed by the shared ancestor of humans and apes which lived a few millions of years ago and from which both the human line and the chimpanzee line separately diverged. Studying living apes can also provide us with a better understanding of the ways in which modern humans are different from apes in some unique ways, and this can also help us reconstruct what were likely some of the most critical steps taken in the evolutionary path to becoming fully human.

Many people observing apes in zoos, on TV, or in the wild can't help but be struck by their obvious physical similarities to human beings, and even by some of their many "almost-human" basic behaviors (the way they play, handle objects, or discipline or comfort a youngster, for instance). To a scientist specializing in anatomy (the form and function of different parts of the body), the similarities are even more evident: most of our basic body parts (our bones and organs) are strikingly similar to those of apes, and this in itself is a big clue as to our shared ancestry. Some of our more apparent differences include: differences in body proportions (humans have relatively shorter arms and longer legs) and the fact that we have relatively hairless skin, a more fully mobile thumb, and a skull that is aligned with our spine in such a way that we can stand, walk and run fully upright (rather than by leaning forward on our knuckles much of the time the way apes do). We humans also have proportionally much bigger brains, and a much more developed capacity for complex language. These are some of our most obvious differences. But, on the other hand, our similarities include the fact that our blood proteins and DNA molecules are almost identical to those of chimps: most molecular biologists agree that there is only something in the order of a 1% to 2% difference between the DNA of humans and the DNA of chimpanzees!

It is known that the longer two lines evolve away from a shared ancestor the more differences will be found in their DNA. Analysis of DNA similarities and differences shows us, for example, that living African apes are more closely related to humans than to any monkeys, a group which separated earlier in primate evolution. Most significantly, the fact that there is still roughly a 98% similarity between the DNA of humans and the DNA of chimpanzees proves beyond the shadow of a doubt that our two species are extremely closely related. In fact, molecular biologists have been able to work back from this data to calculate that we still shared a common ancestor with chimpanzees as recently as about 5 million years ago, which on the overall evolutionary time scale is not very long ago.(See the box "Chimp and Human DNA.")

And then there are the fossils--lots and lots of fossils!

In Darwin's day people were just beginning to look for fossils of possible human ancestors, so nobody quite knew whether many such fossils would ever be found, what they would look like (more like apes, or more like humans, or more like a blend of both?), or whether they would tend to confirm or refute the idea that humans had evolved from nonhuman ape-like ancestors. But in the century and a half since Darwin, many different teams of scientists have been able to collect literally thousands of specimens of fossil hominids of different ages. These include: hominid fossils which are less than 200,000 years old (and which are clearly anatomically modern humans); fossils of a number of different hominid species which are in the range of 1 to 2 million years old (which are not quite anatomically like modern humans but some of which already show some of the critical physical features which distinguish humans from apes and from earlier hominids); and quite a few fossils of different hominid species that lived around 3 to 4 million years ago (which were still very ape-like in many features, but nevertheless were able to walk fully upright).

Hominid fossils that are older than 4 million years old are still very rare. One recent find (from the African country of Chad) is the 7-million-year-old skull, nicknamed Toumai, of what may have been an upright-walking hominid. Various experts in the field are currently reviewing the evidence (including where on the skull the neck muscles attached, which can provide evidence about bipedalism), and not everyone is completely convinced that this creature walked fully upright. However, if this fossil ends up being confirmed as having been bipedal, this find would represent the closest we've yet come to finding remains of hominids who lived at, or very close to, the time of the first divergence of bipedal hominids from a line of African apes. (See the box "Was Toumai One of Our Ancestors?")

Some scientists think bipedalism itself may have evolved (and died out) more than once among the ancestral ape lines. Whether or not that is the case, it is now very clear that the story of human evolution is not some kind of simple straight-line story of how a single long-ago ape species gave rise to a single bipedal hominid species which then went on to produce modern humans. It's much more complicated than that: we now know for sure that, in between those non-bipedal African ape ancestors and modern humans, there have been successive waves of many different upright-walking hominid species.

Some of these bipedal species survived for hundreds of thousands or even a million or more years before going extinct. Some were our direct-line ancestors, and some were more like side branches of our greater hominid family (side-branches which eventually died out without leaving any modern-day descendants). Overall, the pattern of hominid evolution looks much more like a richly branching "bush" than like any kind of straight-line path or "ladder" from ape to human.

Studies of the similarities and differences of various hominid species that lived at different times in the past 5 million years has provided a great deal of concrete proof that modern humans evolved through a series of step-wise evolutionary modifications--starting with the most ape-like of our hominid ancestors, and then unfolding through a series of successive descendant species, many of which had features that were clearly intermediate between apes and modern humans, and then finally producing our own fully modern human species roughly 200,000 years ago.

Do we now know everything that there is to know about the earliest origins of human beings? Of course not. It sometimes seems that every time you open a newspaper there is some report that a team of scientists has discovered yet another fossilized hominid skull or some other part of the skeleton of some hominid ancestor species that lived millions of years ago. And every time this happens, all the different teams of scientific experts in the field understandably get very excited, and then spend months (and often years) verifying the fossil's age and closely examining its features for similarities and differences with modern humans, with living apes, with any hominid fossils found in rock layers of similar age, and with all the older and younger hominid fossils previously found. In this way, information is gathered which makes it possible to figure out (often after much vigorous debate between different scientific teams) exactly where a particular fossil hominid fits into the overall family tree, relative to all the other fossil hominids that have been previously uncovered.

Again, back in Darwin's day the idea that humans were descended from ape-like ancestors was still just an untested idea. Darwin and a number of other scientists were pretty sure this is what had happened, basing their informed speculation on: a) the fact that humans had many anatomical similarities with apes; and b) the fact that all the other forms of life on earth were quite obviously the product of the evolutionary modification of various pre-existing species. So Darwin and his friends and colleagues (and T.H. Huxley in particular) suspected that humans would not turn out to be an exception to this rule.

But, of course, the religious authorities of Darwin's day went nuts, as their whole belief system and mode of existence was evidently very threatened by the suggestion that human beings might simply be the product of natural biological evolution, rather than some kind of special separate creation of a supernatural being. Darwin himself was cautious about this, both because he kept getting mercilessly attacked by the religious fundamentalists, the popular press, and others whose beliefs were challenged by Darwin's discoveries, and because he knew that he didn't yet have enough solid evidence to make the case for human evolution the way he could for so many other species.

Darwin knew that, if humans had indeed evolved out of some pre-existing ape-like species, it should be possible to dig into old layers of rock and find fossil bones linking the human line to the ancestor ape- line, but this hadn't yet been done. In fact, before Darwin's time people had not even known enough to begin to look for such fossils. (The very first such fossils to be dug up--the ones we know as Neanderthals-- weren't discovered until the late 19th century, right around the time Darwin's breakthrough work, the Origin of the Species , was published.) But, from that point on, scientists discovered many hominid fossils, transitional between ancient apes and modern humans.

Hominid fossil finds really took off in the 20th century (especially thanks to the pioneering work of three generations of the Leakey family working in Eastern Africa), and have been especially numerous in recent decades. Today the problem is not the lack of fossils but the fact that there are so many , and that new ones are being uncovered with such frequency that it can be a real challenge to get them all properly sorted out in relation to each other.

As we begin the 21st century, we are able to reconstruct a big part of our family tree, but new fossil discoveries of varied species of related hominids keep adding more information to the mix and frequently cause scientists to revise or fine-tune the exact sequence and degree of relationships linking the different hominid species to the older ancestor ape species in one direction, and to the only remaining hominid species (our own) in the other direction, through a multi-million year process of intermediate evolutionary steps. And again, every time a new fossil is discovered, there typically follows months and even years of analysis and vigorous debate among different teams of scientists before some consensus can be reached about exactly where the new find "fits in" in relation to all the other fossils previously found.

To use an analogy, it is a little bit as if we had a huge family photo album--spanning many, many generations--and we had dropped it on the floor so all the pictures had fallen out. Now we're trying to put it all back together in the right order, but this is difficult to do because a lot of information about exactly how everybody was related has been lost over time. It can be pretty obvious which photos are the pictures of the oldest ancestors, and which are the pictures of the most recent ancestors. But in between it can be hard to tell exactly which photos are of really close relatives and direct-line ancestors, and which photos belong to distant cousins occupying side-branches of the family. There might even be a few photos of ancestors nobody can be completely sure about and whose exact place in the family album some relatives might continue arguing about for years! But, with enough time and patience, it should be possible to eventually glean from the pictures themselves (and from associated bits of historical evidence) enough information to be able to put most of the family photos pretty much back in the right order of relationship.

Well, this is just an analogy of course, but that's pretty much what it's like these days as scientists work to fill out the details of the grand Family Album which links ape-like ancestors of millions of years ago to our own modern human species (the species Homo sapiens ) through a diverse bunch of intermediate relatives--the many different transitional hominid species.

To extend the analogy even further, we could say that, when it comes to hominid evolution, "some of the pictures are still on the floor" (there is absolutely no doubt that many more fossilized remains of ancient hominids will be found in coming years); "a few of the pictures are still probably in the wrong place" (improved techniques and new scientific understanding will almost certainly cause us to further juggle the relative relationships of some of the fossil hominid species in coming years); but all in all "many of the pictures are already in essentially the right place in the family album." In other words, it is really important to understand that there is already general scientific agreement and consensus on what are some of the key relationships and distinguishing features which allow us to link the earliest bipedal (upright-walking) ancestor species to modern humans through a series of intermediate steps and critical evolutionary modifications spread out over a few million years.

So let me try to very briefly summarize some of the things we know at this point about "where people came from." We know that there were many different kinds of upright-walking hominid species living over a period of a few million years, and it is clear that some of them were very "successful" (in terms of being able to maintain themselves as distinct species for hundreds of thousands or even millions of years before going extinct).

We also know that there were times when a number of different hominid species lived in the same general time period (and sometimes even in the same geographical area), differing somewhat from each other in some of their particular physical features (tooth and body proportions, brain size, etc.) and differing in some of the ways they related to their environment--such as what kinds of foods they ate (which can be deduced from their fossil teeth) or whether or not they used or made stone tools (and, later on, fire) to obtain and process their food.

We know that hominid evolution was not "bound"to lead to modern humans, only that it did . And we know, absolutely beyond the shadow of a doubt, that our very distant ancestors were an ape species whose later descendants gave rise, on the one hand, to the line which eventually led to the modern African apes (gorillas and chimpanzees) and, on the other hand, to a whole complex series of upright-walking hominids which eventually led to modern humans.

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FOOTNOTES:

1. And we also can (and should) apply the methods and techniques of modern science to gain a better understanding of such things as why human beings created religions in the first place or why many (though certainly not all) individuals continue to feel a "need" for religion.

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2. As discussed earlier in this series, natural selection is a relatively simple mechanism: new features (representing novelties of form or function not present in earlier generations) can emerge in individual organisms simply as a result of various kinds of "reshufflings" of the inheritable genetic variation which was present in their "parent" generation. When such new features happen to provide individuals having those features with a "reproductive edge" relative to individuals who don't have these new features (allowing them to produce relatively more descendants who will in turn be able to survive and reproduce), the new features will, quite naturally, end up being passed on to a greater proportion of the individuals which make up the population in the following generations. In this way, these features will tend to spread throughout an entire population of plants or animals. This will happen automatically, as long as the features in question really are inheritable (can be passed on from parents to descendants through reproduction), and as long as they really do provide individuals with some kind of reproductive advantage so that those having these new features end up (on average) contributing more descendants to subsequent generations than individuals who don't have these features.

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3. "Part 1, Overview": #1157, June 30, 2002; "Part 2, The Evidence of Evolution Is All Around Us": #1159, July 21, 2002; "Part 3: A Few Words About Adaptation": #1160, July 28, 2002; "Part 4a: How Evolution Produces Whole New Species": #1163, August 18, 2002; "Part 4b: More on Reproductive Isolation, Speciation, and the Emergence of Evolutionary Novelties"; #1164, August 25, 2002; "Part 5: Evolution Is a Proven Fact--The Evidence Is Concrete and Comes from Many Different Directions"; #1170, October 13, 2002. The series is also available online at rwor.org.

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4. The Catholic Church has never had a good track record when it comes to recognizing and appreciating the advances of science: The astronomer Copernicus first proposed that the earth orbits around the sun (rather than the other way around) way back in the 1500s. His theory was then scientifically confirmed to be true by the astronomer Galileo in the early 1600s. Outraged that the scientific evidence revealed errors in the Bible, the Catholic Church baldly refused to accept this scientific evidence and instead persecuted Galileo as a heretic until they could force him to "recant" (take back) the Copernican theory. This was in 1633. The Catholic Pope did finally get around to giving a speech in which he admitted the Church had been wrong and that Galileo had unjustly suffered at the hands of the Church. But this didn't happen until1979!!!

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Because the Bible says that God created all living creatures separately and at the same time, religious fundamentalist Creationists will do anything to try to deny the clear scientific evidence that different species evolved one out of the other and at very different times in the 3-1/2 billion year history of life on earth. They are especially bothered by the undeniable evidence of evolution in the human line, which includes so many fossil species of different upright-walking hominids which are clearly intermediate between ancestral apes and modern humans. The Creationists can't deal with this evidence, so they usually resort to saying that the early upright-walking Australopithecines are "just apes"; that the later hominids like Homo habilis, Homo ergaster, Homo erectus , etc.--which have features clearly intermediate between the more ape-like but upright-walking Australopithecines and modern humans--"must just be fakes"; and that the fossilized remains of our current species, Homo sapiens , going back more than 100,000 years, are admittedly modern humans, but they can't possibly be that old!

Twisting the facts to fit their religious fundamentalist convictions is a Creationist specialty. Until recently, most of them even used to insist that humans and dinosaurs lived at the same time (which would have to be the case if the Biblical creation story were true). The Creationists pointed to human footprints found in the very same rock layers as some dinosaur footprints as supposed "proof" that humans and dinosaurs had coexisted. But there's just one little problem with this: while the dinosaur footprints are real (dating back to before the last of the dinos went extinct 65 million years ago), it turns out the human footprints were simply chiseled into the same Texas rocks during the Depression in the 1930s, when this was a popular carnival game and tourist diversion in that part of Texas. Needless to say, most Creationists don't talk about this much anymore.

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Starting in the 1970s, various teams of scientists have used the techniques of molecular biology to compare human and chimp blood proteins and DNA and have discovered a remarkable amount of similarity between chimp and human DNA at this underlying molecular level. Most scientists working in the field repeatedly came up with the same basic figure of around 98.5% to 99% similarity. Since it is known that the longer two species are separated in evolutionary time, the more their DNA accumulates differences, it has been possible to calculate from this DNA data that modern chimpanzees and modern humans still shared a common ancestor as recently as approximately 5 million years ago. This is not long ago at all, given that life on earth has been evolving for 3-1/2 billion years.

Recently the press has been full of headlines announcing "chimps and humans not as close as previously thought!" This may sell newspapers, but unfortunately it may mislead some people into thinking evolutionary biologists now disagree about whether we are in fact closely related to chimps and evolved from a common ancestor. There is no such disagreement. The headlines simply refer to the fact that Roy Britten, a biologist at Cal Tech, used a new technique to recalculate some of the molecular differences between human and chimp DNA. He used not just the standard assessment of DNA differences measured through nucleotide substitutions, but also factored in calculations of the amount of so-called "indels" (insertions and deletions) of additional DNA fragments which seem to occur particularly in the sections of DNA generally thought of as "non-functional." With this new method (whose overall significance and validity will have to be further evaluated), Britten still comes up with a figure of at least 95% DNA similarity between humans and chimpanzees. (By contrast, for instance, humans and their more distantly related mammalian relatives the mice, have less than 60% DNA in common.)

So, whether you prefer the 95%, 98.5% or 99% figures when describing the genetic relatedness of humans and chimpanzees, it remains absolutely clear that humans and chimps are indeed extremely closely related,and that chimpanzees remain by far our closest relatives among currently living species. (We were even more closely related to our various hominid ancestor species, but they are all extinct now.) Roy Britten himself points out that "a large number of these 5 percent of variations are relatively unimportant," and adds that these figures continue to support the conclusion that the human line and the chimp line diverged from a common ancestor roughly 5 million years ago.

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Was Toumai One of Our Ancestors?

The term "hominids" includes all the species that are considered to be more closely related to humans than to chimpanzees. We know for sure (from the molecular DNA evidence) that bipedal (upright-walking) hominids split off a bit more than 5 million years ago from the line of African apes that also gave rise to the modern chimpanzees. Thanks to an abundance of fossils, we also have quite a bit of information about the many upright-walking hominid species (and there were a lot of them!) that lived in Africa beginning around 3 to 4 million years ago. But fossils older than 4 million years or so are few and far between, in part because populations of the earliest bipedal hominids would necessarily have been small, and because environmental conditions at the time were wetter and not as conducive to fossil formation.

Recently a 7-million-year-old skull was discovered in the Sahara, far from the usual sites of early hominid fossil discoveries in eastern and southern Africa. It has been dubbed Sahelanthropus tchadensis and nicknamed "Toumai." Its discoverer, Michel Brunet, as well as some other experts, are convinced that Toumai walked upright because of some features relating to where its muscles attached to its skull. If this is right, Toumai would represent the oldest bipedal hominid ever found and could literally be the first bipedal species to diverge from a non-bipedal ancestral ape line. However, some scientists are currently expressing some doubts that Toumai's skull proves conclusively that it walked upright like the hominids of 3 to 4 million years ago. And even if it did, it may or may not turn out to be a direct-line human ancestor, since many hominid species ended up being side branches of the human family and evolutionary dead-ends.

Regardless of whether the scientific consensus ends up concluding that Toumai was fully bipedal or not, this discovery is important, including because it provides proof that it is possible (though difficult) to find hominid fossils that are more than 5 million years old, and that at least some of these fossils may be found in parts of Africa which are far removed from the parts of eastern and southern Africa that have so far produced most of the hominid remains.

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