Ian Tattersall: the humans we left behind

UNESCO Courier, Dec, 2000 by Amy Otchet

A world renown paleoanthropologist cuts down old notions of our family tree to reveal a host of unknown ancestors: extinct human species

Like most people, I was taught to think of human evolution as a linear chain, with a "missing link" connecting apes and a series of prototype humans in a process of perfection reaching the pinnacle that we occupy today. This is the traditional view of paleoanthropologists, veritable human fossil hunters who try to piece together our history. But the field is now increasingly divided and you are seen as the leader of a new and growing camp. Please explain.

This notion of human evolution as being a linear trudge from primitivism to perfection is totally wrong. I came to paleoanthropology from the study of lemurs [monkey-like primates] in Madagascar where you have a huge diversity of animals. You cannot help asking, "How did these creatures become so diverse?" Yet this question is not asked in paleoanthropology because there is only one species of humans today. Somehow we believe it is normal and natural for us to be alone in the world. Yet in fact, if you look at the fossil record, you find that this is totally unusual--this may be the first time that we have ever had just one species of humans in the world.

We have a history of diversity and competition among human species which began some five million years ago and came to an end with the emergence of modern humans. Two million years ago, for example, there were at least four human species on the same landscape. Maybe they got along by basically ignoring each other or even having peaceful interactions. We don't know.

In any event, we are now the sole surviving twig on a big branching bush produced by this process of evolutionary experimentation. We're not the pinnacle of a ladder that our ancestors laboriously climbed.

How do your views on human evolution differ from traditional Darwinian notions?

According to Darwin, you have legions of organisms that over time evolve themselves into a new species. It's like a fine-tuning process, guided by natural selection, in which the individuals best-adapted to their environment reproduce and pass on their "favourable" characteristics, so that each generation improves upon its predecessor.

So we tend to think of evolution in terms of characteristics, rather than species. For example, we speak of the "evolution of upright walking" or the "evolution of the hand," often without realizing that legs and hands can only be part of the story. The reality is that natural selection is a blind mechanism which can vote up or down only on entire organisms, warts and all. Individual organisms are mindboggingly complex and integrated mechanisms: they succeed or fail as the sum of their parts, not because of a particular characteristic.

It's the same with populations and species. Species are out there competing with others in a real world of limited resources. What's more, the ecologies of which they form a part have an alarming tendency to change abruptly. If your habitat is covered by an ice sheet, it's pretty irrelevant how well you are adapted to the meadows and forests now buried beneath the ice.

Finally in the Darwinian notion you have a slow accumulation of changes over generations leading to the creation of a new species [when individuals of the same lineage can no longer reproduce]. However a population will change morphologically [biologically] with time but this anything. You're doing your best to reconstruct evolutionary history and you know that science in general is a system of provisional knowledge--it's not an authoritarian system of belief where you make "a" discovery and that stays definitive for the rest of time. All that scientists know is that what we believe today is probably not going to be what we believe tomorrow. Science is grounded in doubt.

But some people are more willing to accept this provisional nature than others. You probably have colleagues fuming with your ongoing list of human species.

It's just because they're not used to it. It's hard to convince people to reconsider fossils they thought they have known for over 30 years. But they are more flexible with new fossils because they don't have any received wisdom about species no one knew existed before.

This seems to be the heyday of geneticists. There is a strong feeling that if they can just manage to extract the DNA from human fossils, we will finally get to the bottom of our evolutionary history. Do you think genetic research will transform the field of paleoanthropology?

My feeling is that the two sets of data are still pointing in the same general direction of multiple human species. And that is comforting. But there are paleoanthropologists out there feeling depressed and saying, "Oh God! Our data don't have the resolution that molecules have and we should always believe molecules above morphology." But I don't think that is true. There are no magic bullets. We can widen the field with genetic and isotope studies but we will learn the most by persuading people to look more closely at the existing fossils.