Human Evolution

by Mathew Iredale

One of my favourite books as a child was a huge volume devoted to the natural history of dinosaurs. It was especially fascinating as it included a brief section towards the end about life after the dinosaurs, and it was from this that I first learnt about our own evolution.

At the time, it all seemed wonderfully simple. Our own species, Homo sapiens , evolved over many thousands of years from the more brutish Homo neaderthalensis , which in turn evolved from the smaller Homo erectus , which in turn evolved from the even smaller Homo habilis , which in turn evolved from a much more primitive ancestor known as Australopithecus , so far removed from ourselves that it did not even deserve the title Homo , the poor thing.

This traditional view of human evolution as one of linear progression is still popular today – at least, among the general public. And also, I have no doubt, among a fair number of philosophers as well. There is something very attractive about the idea that we, Homo sapiens or “Wise human” with our science, philosophy and art, are the pinnacle of evolution; that a natural progression has taken place from brutish ape to modern wise human, each one replacing the last until we reach the final, perfect form.

But this simplistic view of human evolution is seriously flawed. In the first place, there are not just a few species of humans, but numerous species. It is becoming ever more apparent that human evolution is like a bush, not a ladder. Secondly, the idea that human evolution always involves “progress”, in that primitive humans are permanently replaced by more advanced humans, has been put in serious doubt by recent research.

The linear view of human evolution began to be abandoned in the 1970s as different species of humans were discovered that made the linear concept increasingly unlikely. More recent discoveries have only strengthened this position and indicate that not only is human evolution a bush, it is an unruly bush. The four Homo species mentioned above have been joined by H. ergaster , H. antecessor , H. heidelbergensis , H. georgicus , H. rudolfensis and, most recently, H. floresiensis , not to mention numerous specimens which have yet to be properly classified. As the anthropologist Ian Tattersall says of the numerous examples of H. habili- related humans that have been discovered, “The latest suggestion is to remove most or all of them from Homo and to place them into Australopithecus instead. I'm happy to go along with this, although I must note that, while tidying up the concept of Homo , this makes the notion of Australopithecus even messier than it was before.”

The idea that human evolution is linear is clearly wrong, but what about the idea that human evolution involves progress? The idea is that, numerous though they may have been, primitive humans have always been replaced by more advanced humans until we reached the final, perfect form – the modern human.

Although it may at first seem obvious – after all, here we are, modern man, the sole human species – the idea that human evolution is progressive has in recent years been increasingly undermined by the evidence. But it is the publication of two very different pieces of scientific research in the last few months that has been most decisive. The first of these involved the genetic analysis of lice, which sounds unremarkable, but has extraordinary implications. The second involves one of most astounding scientific discoveries in the last fifty years, a new species of human quite unlike any other.

As they evolve in tandem with their host, lice, like many parasites, can be used as unique markers to investigate their host's evolutionary history, something which can be very useful in the absence of any host data.

Of the two types of lice found on humans today, one lives on the body or the head of people all around the world, while the other is found only on the head and is unique to the Americas.

David Reed of the Florida Museum of Natural History and colleagues analysed the genetic differences between the two and found that they were so large that they must have diverged some 1.18 million years ago, around the same time that our ancestors diverged from an archaic H. erectus species.

Reed and his team believe that the louse found only in the Americas was the one that evolved on H. erectus while the worldwide louse evolved on the population that eventually became H. sapiens . As both lice are now found on H. sapiens, at some point in the recent past the two species must have made direct contact again (human lice cannot survive off the body of a host for more than 24 hours) and the H. erectus louse jumped back onto a population of H. sapiens which subsequently colonised America. The question is, when?

According to Reed, the evidence suggests “that H. erectus was contemporaneous with modern H. sapiens in eastern Asia, as suggested by Swisher et al.”

This is a reference to Carl Swisher of the Berkeley Geochronology Center who dated the soil in H. erectus skulls from Java in 1996 and proposed that H. erectus survived in Java up until 53,000 to possibly 27,000 years ago, give or take a couple of thousand years. In other words, H. erectus may have survived on Java at least 250,000 years longer than on the Asian mainland, and perhaps 1 million years longer than in Africa. Swisher raised the possibility that H. erectus overlapped in time with modern humans in Southeast Asia, but he lacked any evidence of direct interaction. This evidence has now been tentatively provided by Reed at al.

So, far from being replaced by more “advanced” species, such as H. neaderthalensis and H. sapiens , it would appear as if H. erectus flourished for thousands of years after the appearance of these species and was walking the earth until perhaps 25,000 years ago.

As if that were not incredible enough, it was recently announced by Peter Brown of the archaeology and palaeoanthropology department of the University of New England that a new species of human has been discovered in Southeast Asia, on the island of Flores. The evidence suggests that H. floresiensis , as it has been named, may have lived up until some 14,000 years ago. To put this in perspective, that is some 3000 years after the caves were painted at Lascaux by modern humans and some 25,000 years after modern humans first reached Australia.

H. floresiensis is thought to have descended from an ancestral H. erectus population. More significantly, as it was only one metre tall, and slightly built, it represents an example of dwarfing in human evolution. According to the authors of the research: ‘ H. floresiensis shows that the genus Homo is morphologically more varied and flexible in its adaptive response than previously thought.'

What this tells us is that only 30,000 years ago there may have been four distinct human species walking the planet. Not one, as there is now, not two, as was thought to be the case up until very recently ( H. neaderthalensis and H. sapiens ), but certainly three, and very possibly four human species walking the earth at the same time. And yet over the next 15,000 years all but one of them died out.

One could not wish for a clearer indication of the role of chance in the survival of our species. If things had gone only slightly differently in the recent past then modern humanity might not have been represented by H. sapiens , but instead by H. neaderthalensis or by H. floresiensis or possibly even H. erectus .

Or perhaps there would have been no representative at all.

Suggested Reading:

‘A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia,' P Brown et al, Nature, volume 431 (28 October 2004)

‘Genetic Analysis of Lice Supports Direct Contact between Modern and Archaic Humans,' Reed et al, PLoS Biol (November 2004, www.plosbiology.org)

The Monkey in the Mirror (Essays on the Science of What Makes Us Human) , Ian Tattersall (Oxford University Press)