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Unit 9: Human Evolution
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Ian Tattersall, PhD

Ajit Varki, MD
Ian Tattersall, PhD, is Curator in the Department of Anthropology at the American Museum of Natural History in New York. He is recognized as a leading scientist in the field of Human Evolution, with a career than spans several decades and countries around the world. Tattersall has authored many books, including Becoming Human: Evolution and Human Uniqueness, and is credited with a number of major exhibits at the AMNH, in particular the Hall of Human Biology and Evolution.


Could you define the word phylogeny?

Phylogeny is evolutionary history. Every group has an evolutionary background, out of which it has emerged, and that is its phylogeny.

Describe the genus Australopithecus.

Australopithecus is a genus of an extinct human precursor. All of nature is divided up into species, and species are assembled into genera, and there are several genera in the hominid or the human family of which Australopithecus is one.

And what do you mean by Hominid?

A hominid is a member of the group that contains Homo sapiens, our own species, and those species in the fossil record that are more closely related to us than they are to any living ape.

Can you describe Mosaic evolution?

Mosaic evolution is a concept that embraces the notion that characteristics accumulate in the phylogeny at different rates. I don't myself think it's a very useful concept.

What about Genetic drift?

Genetic drift is a evolutionary concept, a genetic concept, that states that changes in genes and gene pools can arise for random rather than for adaptive reasons.

What have been the most significant fossil finds?

Well, the last two decades of the 20th century provided a huge number of extraordinary finds of extinct humans that have totally changed our perception of our evolutionary past. The finds range from the middle 1980's with the discovery of the famous Turkana boy skeleton, usually referred to these days as Homo ergaster (or Homo erectus), which is the first modern proportioned being in the human fossil record, through such findings as the earliest immigrants from Africa, the extraordinary finds from Dominisi in Georgia, and recently to this new Sahelanthropus find just reported from Chad in Western Africa [from the French group of Michel Brunet, see Nature, July 11, 2002]. These finds have pushed backwards in time the human fossil record and have also enlarged its morphological scope and its geographical scope. It's been an extraordinary period of developments.

Which of these has been most surprising?

Well, the most recent findings have been a third human skull from the site of Dominisi in Georgia in the Caucasus, dated to about 1.7 million years ago. And it's the first finding of a hominid outside Africa that looks truly African in the sense that it resembles something that is already known from Africa, and it quite closely resembles something that has already been reported from Africa.

Have these finds complicated evolutionary theory?

Well, I think the cumulative effect of the new findings of the past couple of decades have reinforced the notion that human evolution is a matter of evolutionary experimentation rather than of a fine tuning within a lineage. Our "received wisdom" about human evolution at the beginning of this period was that human evolution has been a kind of a linear process, kind of almost inevitably directed towards ourselves with a honing, if you please, adaptations within a single lineage that eventually resulted in Homo sapiens.

But clearly, that was not the case. Clearly human evolution has proceeded in very much the same way as the evolution of all successful groups of mammals, which is to say that new species are continually produced, are thrown out into the ecological arena and are triaged, as it were, by competition among themselves and among other elements in the environments they inhabit. And this process involves a lot of extinction as well as of species origination.

So the idea of a linear human evolution is not correct?

Well, the human family tree as it is increasingly emerging is very bushy in the sense there are many branches, many little twigs, many side branches, rather than a single slender trunk that's sort of proceeding in a majestic way towards the apex.

The bushy notion of human evolution is that the human family tree represents a rather bushy confirmation, with many side branches going off in different directions and many terminations as well as many beginnings. This is very different from the notion of the family tree as being a slender trunk rising towards the apex, and it's very clear at this point that our human family tree has been very bushy in its structure.

Is that an idea that has been suspected for a long time? I personally suspected this for a long period, but it's only really become absolutely apparent in the last 20 years, and with the new findings over that period, this bushy structure has characterized human revolution.

Beginning at about the 1950s the received wisdom in paleoanthropolgy was that human evolution had been a sort of single-minded slog from primitiveness to perfection, if you wish, without a lot of different species being involved and with species tending to evolve from one into another in a kind of a linear fashion. Clearly these new findings over the last 20 years just cannot be accommodated within a structure like that.

What are the biggest frustrations with the human fossil record?

The human fossil record by now is much better than it was even a short time ago, and in fact it's a pretty good record in comparison to many other families of mammals, even though we do have this tendency to bemoan its incompleteness. Of course every fossil record is always incomplete. We'll never have representatives of every species that ever existed. And if we did, we certainly wouldn't have representatives of those species in all the time periods that they inhabited or in all the areas geographically that they inhabited.

The human fossil record, like every fossil record, is always going to be incomplete, and that will carry along with it its own frustrations. But on the other hand, it is absolutely wonderful how our knowledge of human evolution through the fossil record has expanded in the last half century in particular and over the last 20 years.

Let's touch again on mosaic evolution.

The notion of mosaic evolution is a notion that was designed to accommodate the fact that different characteristics, different new novelties, if you wish, evolutionary novelties, emerge at different times in the evolutionary story.

I don't think it's particularly useful actually to think in terms of characteristics and their emergence, because all characteristics are embedded in species, and those species are complex combinations of many, many different characteristics, so I think it's very, very important for us to remember that our evolutionary history is a history of a species, rather than history of disembodied characters.

How about the concept of adaptive radiation?

Adaptive radiation is the notion that when a species enters a new ecological zone, for example, it has the opportunity to exploit this new zone in many different ways, and it's very true that once a new group of animals has become established, it is very common that many, many species emerge within that group within a relatively short time of them exploring the different ways there are to be a species belonging to that group. And that's the basis of adaptive radiation, which is a sort of an explosive diversification of a newly established group.

Human evolution is a wonderful example of adaptive radiation in the sense that it seems now that as soon as early hominids had emerged, early putative bipeds had likewise emerged. They began to explore the possibilities of being upright bipeds in a hominoid kind of a way in many different ways. So human evolution is a very good example of adaptive radiation within a new family of mammals.

When did the dispersal of humans from Africa occur?

Well, we've learned a lot about the dispersal of humans out of Africa over the last couple of million years in the last decade or two. Now, it's quite evident that early humans exited Africa right after the emergence of the modern body skeleton. As soon as you have an upright, striding biped, you have a species that is sort of emancipated from the forest edges to which its precursors had previously been confined. As soon as these new kinds of hominid could move out onto the savannahs away from the trees, they began their history of Wanderlust, which has basically characterized the group ever since.

At two million years ago or so, you begin to find the first hominids of modern body structure and almost immediately they're out of Africa and they have moved possibly all the way to eastern Asia within a very, very short period of time.

So we have evidence of this now in terms of a wonderful fossil record coming out of the 1.7 million-year-old site of Dominisi in Georgia in the Caucasus. We have indications from as far away as Java and China at about 1.8 million years ago that humans had moved that far. And we have several archeological occurrences in the intermediate area that suggest that early hominids pass through those zones too.

So the picture is really beginning to emerge very forcefully that there was an early exodus from Africa immediately following the emergence of modern body form, and a very rapid spread following that to the very eastern end of Asia.

Can you describe the Replacement hypothesis?

Populations are replacing each other all the time. There is a tendency for environments to change rather rapidly, entailing the displacement of animal populations of species from one environment to another, or from one geographical location to another, and there has been a regular history of replacement of foreigners and of species in the fossil record over evolutionary history, so this is a very important mechanism that operates to affect overall evolutionary outcomes, and there's no doubt that the humans are no exception to this.

How does that contrast with the Multiregional hypothesis?

There are two main schools of thought when it comes to the origin of modern Homo sapiens, which is what we're referring to when we talk about "Multiregional" notions or "Out of Africa" notions. And they feed back into notions of the evolutionary process and how that works. The multiregional notion is essentially a notion of continuity within a single lineage, within which fine-tuning is taking place in slightly different directions, but where unity is always assured by the exchange of genes between contiguous populations.

The "multiregionalist" would claim that Homo sapiens has been on the earth for two million years, and has remained a unity even as it diversified in that long period of time. The "Out of Africa" notion, the "single" notion as I would prefer to put it, is a notion based on the idea that evolution is much more complex than simple fine-tuning within a lineage would allow. There are a lot of things going on in evolution. There are many new species being born. There are many species becoming extinct. There are many experiments with different ways of doing evolutionary business, and with the notion that in order to have a new species you have to have a single point of origin.

At present it looks most likely from what we know in the fossil and archeological records that the point of origin of our species, Homo sapiens, lay in Africa. Hence, the notion of the "Out of Africa" hypothesis, whereby Homo sapiens emerged somehow in Africa at some point over about a hundred thousand years ago, and subsequently spread from Africa to all parts of the world, and then that species has subsequently diversified in the way that all species do that inhabit very large tracks of geographical territory.

What about so-called evolutionary dead ends?

Evolutionary dead ends are very common in the human fossil record. In fact, there are people who would claim that we probably don't know any direct ancestors to Homo sapiens in that record. But if the pattern of human evolution has been one of the production of new species and the selective extinction most species in the fossil record, then clearly many, many species that we know as fossils were evolutionary dead ends in the sense that they didn't give rise to descendent species.

Are the Neanderthals such a dead end?

The extinction of the Neanderthals is a perennially fascinating question for all human beings, because it's the best example we have of a resident group of distinctive hominids-a separate species, not Homo sapiens-being invaded by an incoming population of Homo sapiens, and then disappearing as a result.

Has that idea been hard for people to accept?

Well, for a long time the dominant notion of human evolution was a linear notion, whereby one species gave rise to another, to another, to another, and eventually Homo sapiens emerged at the top of the heap. In that case, you have to fit in the Neanderthals as a linear ancestor to ourselves, but it's very clear that the Neanderthals were so different from us that they could not possibly have been linear ancestors. They were our "co-lateral relatives," as it were.

And in Europe you have the very good record of a stable, successful species, Homo neanderthalensis, as an ancestor being invaded by an entirely new kind of hominid called Homo sapiens, and disappearing very shortly after the invaders arrived.

This maybe gives rise to some sort of feelings of guilt. Did we make the Neanderthals extinct? Are we really that nasty that we could do that? But in fact extinction and replacement of populations and foreigners is a very routine thing in evolution. The extinction of the Neanderthals is not something we should be particularly surprised about, and it doesn't tell us anything about our essentially darker side than looking around at ourselves in recorded history.

What about the extraction of DNA from fossils as a tool?

It's a wonderful thing that it has been possible to extract small stretches of DNA from Neanderthals who have been dead 40,000 years. It's a wonderful tour de force of technology, and it's also reinforcing our perception that the Neanderthals were very, very different from us. The Neanderthals lie completely outside the envelope of modern human populations in terms of the mitochondrial DNA that has been extracted.

And how about other types of molecular data?

Molecular techniques have added a great deal to the armamentarium of approaches that paleoanthropologists can now bring to bear on the history of our species, Homo sapiens. One way, of course, is by the direct extraction of DNA from ancient humans, which has so far only been done with Neanderthals that lived in pretty cold climates in which such delicate molecules as DNA tend to be preserved. And what that particular work has done is to draw attention to the fact that the Neanderthals are indeed very distinctive from ourselves, and it has given us some way of approaching the date of divergence between the Neanderthals (or the lineage leading to Neanderthals) and the lineage ultimately leading on the other hand to Homo sapiens. That's one way in which molecular techniques have really helped us.

The other way is in terms of trying to decipher the history of human populations from a single origin, probably in Africa. Homo sapiens has moved to every continent of the world and has diversified in those continents just as all successful populations tend to do. There are slightly different evolutionary histories in different areas, the emergence of slightly different-if only superficially different-novelties in different geographical populations. And the history of those populations can be studied by traditional means, but also in a very powerful way by the addition of molecular techniques. There is a tremendous amount of information about historical processes of diversification that is held within the genetic code, and this has been a very fruitful new area for understanding the history of the human species.

How has mitochondrial DNA been used as such?

Mitochondrial DNA has been particularly interesting in regards to reconstructing the history of human populations because mitochondrial DNA is inherited strictly through the mother. Nuclear DNA gets jumbled up in every generation as the DNA of the two parents of each individual is recombined. But this doesn't happen in mitochondrial DNA, and so you have a much clearer record of the accumulation of small differences in the mitochondrial genome.

Can you touch on the concept of Mitochondrial Eve?

Mitochondrial Eve is a common ancestor of all human beings. We know that our species must have originated in a small population, because that's the only way in which you can get the similarities in mitochondrial DNA that we all share today. Mitochondrial Eve is a notional common ancestor of all living human beings, an idea which is created by tracing all of the various lineages of mitochondrial DNA that we know today back to their common ancestral form. Eve might not necessarily have even been a member of Homo sapiens by the way. It's possible that all of our modern strains of DNA actually converge at an earlier point, but it's a useful concept for deriving the notion of a common ancestor of everybody who belongs to Homo sapiens.

What about the popular confusion that Mitochondrial Eve is the one traceable woman who was the 'mother of us all'?

It's important not to literally confuse the "mitochondrial Eve" with the "biblical Eve." Two different stories with the same intent, of course, are being told here, and they're not directly comparable.

Nonetheless the concept of Mitochondrial Eve, the concept of our ancestry as lying, if not necessarily in a single individual-clearly it cannot lie in a single individual-but within a small population, is crucial to our understanding what the origins of our own species were.

And what would that population size have been?

It has been clear for a very long time that big populations just have so much genetic inertia within them that they are very, very resistant to change. True evolutionary novelties can only really arise within very small populations, so whether there is enough genetic instability for novelties to become fixed or established, it is very important to bear this in mind. You can't just march a large population along an evolutionary trajectory in a single direction, simply because there's just too much inertia against that.

How does the idea of a molecular clock tie in to this?

The molecular clock is useful within a restricted sense. The molecular clock is based on the notion that change in the molecules that constitute our genetic code is pretty much independent of circumstances, except for time. The change is in sort of a clocklike fashion, ticking away over time. This may or may not be an accurate assumption in particular cases. The assumption of regular change over time is more likely to be accurate in the case of mitochondrial DNA, which apparently does not code for particular characters that may be affected by adaptational or selectional concerns, than in the nuclear DNA complement, where you do have notions of adaptation and selection entering into the picture as well as the rate at which genetic substitutions accumulate.

What would you add about the use of the 'molecular clock' to estimate the age of our ancestors?

Well, back in the early 1970s, paleontologists generally reckoned that the human ape split must have gone back to 12 to 14 million years ago or so. And then, on the basis of molecular studies done by immuno-geneticists, it was estimated by Vincent Sarich and Alan Wilson (of the University of California at Berkeley) that the common ancestry of apes and humans could not go back beyond five millions years, and I think Sarich said no fossil can be considered to be hominid that is over five million years old, no matter what it looks like.

Well, clearly now we have fossils that are older than five million years that are compatible with being members of the hominid family of the human family in the very largest sense.

So clearly that early estimate of the molecular clock was a little bit off. But it was a salutary change in perspective towards a more recent origin, and nowadays most paleontologists probably estimate that the human and ape families split from a common ancestor that existed at maybe seven million years ago or eight million years ago-something like that.

Can you comment on the idea that different protein expression in brains of chimps vs. humans, rather than DNA sequence, is responsible for the differences between us?

Yes, it's very, very exciting now that we're moving beyond this notion of genomics to this notion of proteomics. Svente Paabo and his colleagues, particularly, working with apes have been able to open new doors, not just on the nature of the genes themselves but on the nature of gene expression and the processes whereby the genes are expressed in the development of individuals, and it turns out that there are substantial differences in gene activity, and in systems so far studied between apes and humans and that's very exciting. This is something that we cannot ignore by just looking at the structure of the genes themselves.

What about your recent research?

Well, my latest research has involved an effort to try to document every human fossil that it was possible to get, to study and to produce a series of volumes documenting all of the fossils that have been assigned to our family, using a uniform and standard descriptive protocol. And that's been very exciting. It has involved going to different museums and institutions all over the world and studying many, many, many fossils and the signal that my colleague, Jeffrey Schwartz, and I who have been doing this work together have derived from this survey of the whole human fossil record is the incredible diversity that there is within this record. [Tattersall has co-authored a book with Schwartz documenting these specimens, titled Extinct Humans.]

You know there are now maybe about 18 names, species, of fossil humans, far more than we had thought even 20 years ago. But it's clear that this doesn't begin to express the diversity of fossil species that actually participated in this long seven- or eight-million-year history of human origins.

What do you see as the future of human evolution?

The notion that the process of human evolution has been one of fine tuning is a notion that automatically implies that it's ongoing, and that we can look to the past to predict where our species is headed in the evolutionary future. In fact, if you look around today, you don't find the conditions that are necessary for evolutionary change, for the fixation of evolutionary novelties, in the human population. We've already spoken of the fact that it's necessary to have small populations in which to implant evolutionary novelties. And it's very clear that our species right now is heading in the opposite direction. In fact, our species is becoming more widespread, more densely packed over the face of the earth, individuals are incomparably more mobile than they ever were before, and the conditions for the fixation within small populations of evolutionary novelties just don't exist anymore. So I don't think we can rely on evolution to sort of ride in on a white horse, as it were, and save us from our foibles.

What would you say about the idea of different human races?

Biologically race is a very simple issue. There are two possible mechanisms or processes that happen evolutionarily, speaking within a species population such as Homo sapiens. You can have diversification in small populations scattered over the face of the world, or you can have reintegration of those populations because there has been no speciation, there is no reproductive barrier within the species.

And the inevitable result of that is that in the early days, when humans were very thinly scattered over the face of the earth, there was a certain amount of diversification in local populations as those populations accumulated random differences or slight adaptations to local circumstances.

But since there has been no speciation, since we're all members of the same species, we can reintegrate, too, because these local variants are simply a feature that can disappear by interbreeding, and the more recent history of our species has been one of reintegration.

So, yes, you can see certain general model types, if you want, from different populations with origins in different continents around the world. But we have been reintegrating for so long that any boundaries between those populations have become thoroughly blurred, and there is no biological sense in trying to make discriminations among them. You know, race to the extent that it's an issue at all is basically a social issue, and not a biological one.

What do you most want this audience to know?

I think the lesson that I draw from the contemplation of the human fossil record is that humans are a part of nature. They are intimately linked to the rest of nature. Yet we're clearly different, and that difference is rather recent. I don't think the Neanderthals would have demarcated functionally from the rest of nature in the way that we have.

Novelties of behavior and cognition that make us so different are of very, very recent origin, and in a sense, you know, we haven't had a lot of time to accommodate to this new phenomenon that "we" really are.

What do you see as the most vexing questions in human evolution now?

Wherever you turn in the human fossil record, there are unanswered questions. There's no doubt about that at all. To me there are two particular things of interest. One is the range of total diversity that existed over the long span of seven million years or more of human evolution. And the other question is exactly what it was that was involved with the emergence of modern Homo sapiens, which is so different behaviorally and cognitively from every other species that we know of that has ever existed.

This is a very vexing question partly because it's clear that human beings that look like us were around for quite a long time before human beings who behaved like us, and so we really only have the archaeological record to help us in understanding this emergence.


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