Part 6c: The Clear, and Growing, Evidence of Evolution - From Apes to Human Beings

The Science of Evolution

The Evolution of Human Beings

Revolutionary Worker #1181, December 29, 2002, posted at

Many books and scientific papers have been written on hominid evolution, and new discoveries are adding to our understanding of some of the details and richness of human evolution on practically a daily basis. Here I can only try to give a quick sketch and summary of where our understanding basically stands today.

As I’ve mentioned in previous installments on human evolution, we know for a fact that we are very closely related to the African apes of today (the gorillas and the chimpanzees). We actually have roughly 95-99% of our DNA in common with chimpanzees! This is very significant, because it has been demonstrated conclusively that the more closely related species are to each other, the more similarities can be found in their DNA molecules. By contrast, the more distantly related two species are, the more differences will have accumulated in their DNA and other internal molecules.

As discussed in an earlier installment of this series, scientists have a number of different methods for verifying and cross-checking the age of various things, including fossilized bones. Among the many cross-checking dating methods is the modern technique of molecular dating, which allows scientists to compare the degree of similarities and differences in the DNA of living species, and in turn makes it possible to get a pretty good idea of the time at which two evolutionary lines split off from a shared common ancestor and went their separate ways.

According to this "molecular clock" data, the chimpanzee line and the human line still shared a common ancestor as recently as about 5 million years ago. For a number of reasons that I won’t get into here, scientists generally consider that the chimpanzee line (which today consists of only two species—the common chimpanzee, Pan troglodytus, and the pygmy chimpanzee or bonobo, Pan paniscus) is a relatively "conservative" lineage, which means that it is not thought to have changed too radically since the time of the ancestor it shared with the human side of the hominid line. (Unfortunately, there is no direct fossil record for the chimp line the way there is for the human line, because forest-dwelling species such as chimpanzees live in environments where even bones tend to decay rapidly without forming fossils.) But the DNA evidence, among other things, indicates that it is not unreasonable to consider that the common ancestor of the chimpanzee line and of the human line would in fact have been recognizable to us as an African ape. Like today’s African apes, this ancestor species may well have occasionally moved on the ground by leaning forward on its knuckles (a form of locomotion known as "knuckle-walking"), but it probably spent much of its time using the long arms and grasping feet typical of forest apes to hang and swing through the trees while feeding on the plentiful leaves and fruits of tropical forests.

As I indicated in the last installment, 20 million years ago or so there were many more species of African apes than there are today. But by around 10 million years ago (give or take a few million), their diversity had sharply declined. That’s interesting, because that’s getting pretty close to the time when the molecular evidence indicates the line which led to humans and the chimpanzee line split from their common ancestor. And it’s also getting close to the time when a radically different kind of "ape" starts to appear in the fossil record: the first of the upright-walking hominids! This suggests that whatever factors (such as significant environmental changes) may have been contributing to a reduction in the total number of forest-dwelling ape species at that time may simultaneously have been contributing to the relative "success" (from a reproductive standpoint) of the first offshoot ape species which happened to be bipedal—especially if bipedalism itself facilitated the exploitation of food sources from a greater variety of environments, including those having few or no trees.

As I’ve explained before, it’s always hard (and often impossible) to find fossils of the very first representatives of any distinct evolutionary line, both because of the general problem that most bodies never end up getting preserved as hardened skeletons in the first place, and because most new species probably originate with only small numbers of oddball individuals tucked away in isolated geographic pockets, and only later spread and further diversify into their own descendant species. Today we have many bipedal hominid fossils from 3 to 4 million years ago (when there were already many hominids walking around), but we don’t yet have fossils that we can be sure represent the very first species of bipedal (fully upright-walking) hominids. The 7-million-year-old Toumai skull recently found in Chad is a possible candidate, if it turns out that in fact this was truly a bipedal species.

The fact that the very first species of bipedal hominids would necessarily have started out, like any other new species, with tiny numbers of individuals, makes the task of finding fossils of the oldest bipedal species particularly difficult and means that, for now at least, we have to be mainly satisfied with the numerous (and constantly increasing) finds of the many early bipedal hominids (such as the various Australopithecines) which become relatively common in the fossil record starting around 3.5 million years ago. These fossils show evidence of having features that are clearly intermediate between those of apes and modern humans, with some being more ape-like and some more human-like.

Just to give you a taste of the truly rich diversity of species that have all been identified as upright-walking hominids up to this point (a diversity which will no doubt be revealed to be even more rich and complex as more fossils are found), let me just list some of them, using their currently agreed upon scientific names.

The oldest found so far seems to be Sahelanthropus tchadensis (nicknamed Toumai) from 6 to 7 million years ago. There is not yet general scientific consensus that this particular species was bipedal, so I am including it here only on a tentative basis. I will do the same with the 4.4-million-year-old Ardipithecus ramidus, which cannot yet be definitively confirmed to have been bipedal.

Among the more unquestionably bipedal hominids we find: Australopithecus anamensis from between 4 and 5 million years ago; Australopithecus afarensis (including the famous nearly complete skeleton dubbed "Lucy") from about 3.5 million years ago; Meave Leakey’s recent find of a more flat-faced (and in this way more human-like) hominid from approximately the same period (currently dubbed Platyops kenyanthropus—see box "Meave Leakey’s Take on Her Recent Fossil Find").

Following that we get to Australopithecus africanus, Australopithecus aethiopicus and Australopithecus garhi, all of which lived in the period from about 2.5 to 3 million years ago. Of these, the 2.5-million-year-old Australopithecus aethiopicus (the famous "Black Skull" found by Richard Leakey, and so named because of the beautiful color of the minerals coating the bones) may actually have been the first of a side branch of the Australopithecines which would later include Australopithecus robustus (also known as Paranthropus robustus) and Australopithecus boisei (or Paranthropus boisei)—the so-called "robust" Australopithecines—who lived up to about 1.5 million years ago. These "robust" hominids differed from the so-called "gracile" Australopithecines (such as A. africanus and A. garhi) because they had really massive skulls and jaws, large chewing muscles, and huge grinding teeth. They are thought to have represented an alternative pathway in hominid evolution, one which consisted of species with a more specialized diet of tough plant foods. These "robust" Australopithecines were ultimately an evolutionary dead-end, and they are not considered to be direct-line ancestors of human beings.

Some of the "robust" Australopithecines from about 1.5 to 2.5 million years ago did, however, overlap in time not only with the "gracile" species of the "later" Australopithecines but also with some of the earliest species in our own genus Homo. In fact, in the general period of about 1.5 million years ago there seems to have been six or more different species of bipedal hominids in Eastern Africa, with some of the "robust" species overlapping with some of the early Homo species, which had most likely evolved out of the more gracile lines of Australopithecines.

The new species in the genus Homo were bipedal hominids like their predecessors, but they had many new features, including more human-like body proportions and faces and much bigger brains. At this point, the fossils in the new Homo line include the species Homo habilis (sometimes classified as Australopithecus habilis because of its "intermediate" character) and Homo rudolfensis, which both lived around 1.5 to 2.5 million years ago—corresponding to the period in which manufactured stone tools first appear in the fossil record.

These early Homo species were then followed, right around 1.5 million years ago, by the much more long-legged, big-brained and more modern-like Homo ergaster (also known as the African form of Homo erectus). Homo ergaster/erectus (which includes the famous and much-studied nearly complete fossil known as the Turkana Boy) is the first hominid species known to have made tools as complex as stone axes and to have had fire. It is also generally considered to have been the first hominid species to have spread outside of Africa, in the first great wave of hominid migration. Its descendants eventually reached areas as far away as China and Southeast Asia (where their fossils are generally known as Java Man, Peking Man, or the Asian Homo erectus), but they all eventually became extinct. A variant of Homo erectus also made it to the Middle East and to Europe, where its descendants’ fossils are commonly known as Homo heidelbergensis and Homo neanderthalensis or simply "Neanderthals." They too eventually became extinct.

It may turn out that bipedal hominids were already physically and socially able to undertake far-ranging explorations even prior to evolving the longer bodies, the much bigger brains and the more advanced stone tool and fire technologies which are associated with Homo erectus/ergaster and with our own later modern human species of Homo sapiens. Some people think even some Homo habilis individuals may have begun to migrate out of Africa. (See box "Was Homo Erectus the First Hominid to Leave Africa?")

It will be interesting to learn more about all this as more scientific data comes in, including because the status of Homo habilis—which has features that appear to be intermediate between the more ape-like Australopithecines and the more human-like later species in the Homo line—has always been somewhat controversial. Some people have been inclined to somewhat "demote" H. habilis to Australopithecine status and to question whether habilis really made simple stone tools, or whether those found in rock layers from the time habilis was around were more likely made by a different species of Homo living during the same period (such as Homo rudolfensis). In any case, all this makes clear that there likely was already a lot of variety (and a lot of potential for expansion) even very early on in the Homo line.

Back in Africa, our own fully modern human species of Homo sapiens evolved (most likely out of Homo ergaster or a very similar species) roughly 200,000 years ago. Our new species was clearly quite successful, and by 50,000 years ago was migrating out of Africa and spreading to all other parts of the world. This migration wave was therefore at least the second time hominids had migrated out of Africa.

As Homo sapiens spread into different parts of the world, we encountered and co-existed for some time with populations of different human species, such as Homo neanderthalensis. These Neanderthals were the descendants of some of the earlier Homo erectus populations which had migrated out of Africa 1 to 2 million years earlier. Since Homo sapiens and Homo neanderthalensis overlapped in parts of Europe for a few thousand years (and since some of H. sapiens’ more advanced stone tools have been found at some Neanderthal sites, suggesting that the two species interacted to some extent), it was once thought the two species might have actually mated and interbred and that modern humans could be descendants of both. Today, however, we know that this isn’t the case: modern techniques of molecular biology have been used to analyze the DNA and other biochemical molecules of modern humans all around the world, and this has determined that all living human beings are derived from a single population that lived in Africa around 150,000 years ago, and that the Neanderthals never contributed to the overall gene pool of our species. It is now thought that the hominid line leading to modern human beings and the one leading to the Neanderthals separated about 600,000 years ago; such a long period of reproductive separation could easily have allowed sufficient genetic differences to accumulate to result in two completely distinct species, and this in turn would explain why populations of Homo sapiens and H. neanderthalensis which later encountered each other in Europe (around 40,000 years ago) seem to have been unable to successfully interbreed and produce viable and fertile descendants.

Although Neanderthals were not a direct-line ancestor of our own species, they serve as a good reminder that, as recently as 40,000 years ago, there were still at least two or three different human species on this planet: the modern Homo sapiens (our own species), the Neanderthals in Europe, plus the various populations of the descendants of Homo erectus which had made it all the way to Eastern Asia in that earlier migration wave. But, as Homo sapiens spread throughout the globe, everywhere it went it ended up one way or another replacing these different human species. We know from the fossil record that Homo sapiens had, among other things, more advanced stone and other tools than the other human species around at the time. The more complex skills required to conceptualize and make these tools likely reflect important differences in our species’ cognitive (mental) abilities, and this may explain why it successfully replaced the other human species everywhere it went.

All the other upright-walking hominid species (including the other essentially human species in the genus Homo) are gone now—except for us. But it is important to remember that the mere fact that all those others eventually went extinct doesn’t mean that the earlier hominids were somehow "flawed" or "inferior" species: in fact, a number of these hominid species had a good long run of it, and some, like Homo erectus, lasted for as much as a million years or more. The fact that only our own hominid species is still around is not surprising. All species eventually go extinct, and on average, most vertebrate species don’t generally seem to last more than a couple of million years or so.

Again, the fact that all the other hominid species are now extinct doesn’t mean that "we" are somehow the most perfect species imaginable, or that all the others were somehow flawed or "inadequate." We do have some very unique particularities (such as our unprecedented ability to continually modify ourselves and transform our world through cultural means) which may end up allowing us to have an above average run as a species. However, these same features and "abilities" could also easily lead us to bring about our own extinction in record time, through the use of weapons of mass destruction and/or by causing massive environmental dislocations. Time will tell!

Why Is Our Hominid Species the Only One Still Standing?

The reason all the other hominid species are now extinct has to do with some of the basic laws of biological evolution: the external physical and biotic world in which a species lives and with which it continually interacts (the physical features, such as climate and terrain, and the biotic features, such as the mix of predator and prey species occupying the same environment) is itself always changing, though to a greater or lesser degree at different times. Given this, all species are likely at some point to encounter significantly new "challenges" from a changed environment to which they may or may not be able to adapt through further evolutionary modifications spread out over successive generations.

Populations of organisms which happen to encounter significant environmental changes to which they cannot adapt quickly enough will simply tend to go extinct. But by the time they go extinct, many species will already have spawned one or more descendant "daughter species" (with which they may even have continued to co-exist for a time) which will in a sense continue their evolutionary line. When species go extinct without having produced any descendant species or lineages, they simply become "evolutionary dead-ends." This was the case with some (though obviously not all) of the upright-walking hominid lines and species.

It is important to realize that all biological species eventually go extinct, regardless of how "successful" they once were: It is estimated that something like 99% of the species of plants and animals that ever lived on earth have become extinct! As for our own species, the fact is we’re the only ones of the hominid species left standing not because we’re some kind of "miracle of progress and perfection" and not because there was any kind of inherent driving force in hominid evolution (or in any of the evolutionary processes that came before) that was somehow magically "bound to" lead up to humans as some kind of top of the heap pinnacle of evolutionary "progress." It only sort of looks like that, from an understandably biased and self-centered human perspective! Many biologists have pointed out, for instance, that many much simpler organisms (such as the bacteria and many parasites) have been around for so much longer than humans and are so widespread that they could easily be considered the most biologically "successful" species on the planet!

We’re the only species left standing in the hominid line in part simply because we’re still sort of the new kid on the block, having evolved out of our immediately preceding hominid ancestors (likely Homo ergaster or a very similar species) only around 200,000 years ago. However, now that we’re here, I think we can say that—given our unprecedented ability to consciously transform ourselves and our natural and social environment—what happens to our species in the future (including when and how our own species will pass from the scene) will be, at least to a significant degree, up to us.

What Makes Us So Special, If Only to Ourselves?

Let’s go back to those earliest of the upright-walking hominid ancestors that we generally refer to as the Australopithecines. Many people ask: were they apes or were they people? Well, I think it would be correct to say that they were a little bit of both. It has become rather fashionable in recent years to focus more on their ape-like characteristics, but this seems to me a bit misleading. As a group, the different early Australopithecine species did still have a lot of ape-like characteristics (some species more than others) but many of their fossils also had features which can best be described as intermediate between more typically ape-like features and the later more typically human features. For instance, they had the long arms and short legs of apes, but they did stand and walk upright, as do humans. Their brains were somewhat bigger than those of apes, but much smaller than those of later hominids. Their faces jutted forward in a snout more like chimpanzees than humans, and their teeth and jaws were also more like those of apes. In fact, these early Australopithecines, together with their later descendants, and then followed by the early and late species in the Homo line, constitute one of the best examples (in all of the biological fossil record) of a series of transitional species which can be seen to link one species to another through a sequence of intermediate steps. This—the clear evidence of genuinely intermediate species in, of all things, the human evolutionary line—drives the Creationists nuts, because, if it were true (as is said in the Bible) that human beings are the result of a "special and separate" Creation, there obviously wouldn’t be any such intermediate species linking apes to humans in such obvious step-wise fashion. Well, what can I say? The Creationists are stubbornly and absurdly wrong—the whole long and diverse sequence of different upright-walking hominid fossils proves them wrong way beyond the shadow of a doubt!

What do the Creationists say about the growing numbers of hominid fossils which are staring them in the face?

Well, as the paleontologist Niles Eldredge points out in his very helpful book The Triumph of Evolution and the Failure of Creationism, the response of the Creationists, when they are confronted with the basic facts of human evolution, is downright pathetic.

a) The Creationists generally argue that the fossils of the earliest hominids (the ones which lived around 3 or 4 million years ago) are "just apes." This completely disregards the significance of the fact that these early hominids walked upright, and that at least the "later" Australopithecines made some use of primitive tools, beyond what we see in modern-day apes. Some of the "later" Australopithecines likely were even beginning to make some simple stone tools: Fossils of Australopithecus garhi from around 2.5 million years ago have been found associated with the fossilized remains of antelopes whose bones had stone tool "cut-marks" on them, indicating that they had likely been butchered with stone tools. And simple stone tools have in fact been found in nearby sites.

b) The Creationists argue that the fossils that look the most like modern humans are indeed humans, but they refuse to accept the reality that some of these fossils are 100,000 years old (despite the fact that modern scientific dating techniques leave no doubt that this is the case). The existence of anatomically modern humans so long ago conflicts with the Biblical story of Creation.

c) The Creationists especially reject the evidence that shows that some species of hominids were clearly intermediate between the more ape-like Australopithecines and the fully modern humans: these are the various species in the genus Homo, such as Homo ergaster and Homo erectus, which evolved later than the earliest bipedal hominids but before our own Homo sapiens species. These hominid species are known from the fossil record to have made much more complex stone tools and to have used fire. Measurements also show that their brain size was intermediate between the earlier hominids and fully modern humans. So species like Homo erectus and Homo ergaster are real problems for the Creationists, precisely because they are such obvious evolutionary intermediates between the more ape-like early hominid species and fully modern humans. So what do the Creationists do? They simply declare that all these fossils are fakes!

Since more and more such fossils are constantly being discovered (and by a number of different teams of paleontologists and scientists working in many different localities), it is not clear how much longer the Creationists are going to be able to stick to that particular claim! I can only wholeheartedly agree with paleontologist Niles Eldredge when he writes: "That the best the creationists can do with the human fossil record is call the most recent fossils fully human, the earliest merely apes, and those in the middle—the intermediates if you will—outright fakes is pathetic... The irony is great: the case toward which all their passion for producing propaganda is ultimately directed—how we got here—is about the most difficult one I can think of to support the model of creation."

Meave Leakey’s Take on Her Recent Fossil Find

The paleontologist Meave Leakey is continuing and extending the astounding legacy of the Leakey family (her in-laws were Louis and Mary Leakey and her husband is Richard Leakey), all of whom have made enormous contributions to hominid and other fossil finds and to untangling the story of early human origins in Africa. Meave Leakey had already found some new specimens of 4-million-year-old Australopithecus anamensis (the likely ancestor of A. afarensis, popularly known as Lucy). Then, in 1999, assisted by her daughter Louise, she found yet another fossil hominid skull on the shores of Lake Turkana in Kenya. She dubbed it Kenyanthropus platyops ("flat-faced man of Kenya"—named thus because it had the less ape-like and more human-like flat face that is more often found in later hominid fossils). This is a fairly early hominid who lived about 3.5 million years ago, at around the same time as Australopithecus afarensis (which includes the fossil known popularly as "Lucy"). Leakey is confident that platyops is different enough from afarensis to be placed in a separate genus. Whether this particular species of hominid is an even better candidate for a direct-line ancestor of human beings or will turn out to have been more of a side-shoot species is not yet clear. But, as Leakey points out, what is completely clear is that the evolution of bipedalism early on produced a diverse "bloom of species," which was only later whittled down. Today, Leakey says, our modern human species is "the sole remaining species. We’re one little twig left on the past’s complicated tree."

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Was Homo Erectus the First Hominid to Leave Africa?

It is well known that individuals of a fire-bearing and tool-making hominid species, known variously as Homo ergaster or Homo erectus, started to travel out of Africa beginning roughly 2 million years ago. They eventually reached as far as Asia (where fossils of their descendants include the so-called Java Man and Peking Man) as well as the Middle East and Europe (where fossils of their descendants are referred to as the Neanderthals). These were already relatively "advanced" humans (with relatively advanced stone tools, for instance), but they did not belong to our fully modern Homo sapiens species, which appears to have evolved out of remaining Homo ergaster populations in Africa a bit later—around 200,000 years ago. Homo sapiens began to spread out of Africa about 50,000 years ago and eventually finished replacing the Neanderthals and other descendants of the older H. erectus species throughout the world by about 35,000 years ago.

We know for sure that Homo erectus was an early migrant out of Africa, but was it the first? Recently a number of fossil hominids dated at about 1.7 to 1.8 million years old were discovered in the country of Georgia (between the Black Sea and the Caspian Sea, near Russia, Turkey, and Iran). It is not yet clear whether these fossils (known for now as the Dmanisi fossils) may simply belong to the well-known Homo erectus species or whether they belong to an even earlier species of hominid. They had small brains and chimp-like faces, and they made very simple stone cutters and scrapers. In many ways, they appear to be somewhat intermediate between the older Homo species such as Homo habilis or Homo rudolfensis (which had simple stone tools starting around 2.4 million years ago but which, until now at least, were never thought to have left Africa) and the later Homo erectus species, which was closer to modern humans in appearance than the Dmanisi fossils and had more advanced stone tools, and which we know for sure migrated into Asia and Europe starting around a million years ago. The "intermediate" appearance of the Dmanisi fossils (between H. habilis and H. erectus) may be misleading, and these fossils may turn out to be early Homo erectus after all. But it is possible that they represent the descendants of some earlier migrants, and that Homo erectus wasn’t the very first to travel out of Africa.

Interestingly, the Dmanisi fossils are associated with fossils of some other African animals (such as ostriches and short-necked giraffes): it could be that widespread environmental changes in Africa were encouraging a number of species to migrate around that time. Homo erectus, or whatever hominid species these turn out to be, may have been simply following some of those other animal migrants out of Africa. Perhaps some similar factors later prompted our own Homo sapiens species to extend its range and spread to other continents, as we know it did starting around 50,000 years ago. We’ll no doubt learn more about all this as more scientific evidence comes in.

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