Saturday, August 4, 2007

Trunk-Less Tree of Life

No story of origins has captivated and excited humans more than the evolutionary origins of humankind. The recent discovery of a 6-7 m.y old hominin fossil nicknamed Toumai, from Chad Africa has added to this excitement. The recent editorial, titled Tree of Life in the Times of India, 16 July 2002, does a fair job describing the discovery but also exposes some basic misconceptions about the nature of evolution. I’ve listed some of the misconceptions below.-

…“could help link the huge evolutionary gap between the 10 million-year-old ape age and the five million-year-old find, believed to be evidence of the first hominids”.

This would mean the huge gap in the fossil record of African apes between 10 million and about 5 million years ago. There is off course no “evolutionary gap” between 10 my and 5 my ago. Apes continued to evolve.

…”the new-found skull, Sahelanthropus tchadensis, is thought to represent that point on the evolutionary time-scale when hominids took shape, moving away from being chimps but still a long way off from today’s humans”.

Humans and chimps shared a common ancestor 7 million years ago. Chimpanzees did not exist 6 million years ago. This early hominin fossil shows enough distinct morphological traits that suggest that it represents the lineage that lead to humans and not the lineage that lead to modern chimpanzees.

…”We need to find our way through these various branches (of evolution) which have their own distinct characteristics, but which are ultimately connected to the main trunk — Homo sapiens”.

This to me represents a deep misunderstanding of the evolutionary process. The article rightly points out that evolution is more like a branching tree than a simple ladder like succession of species, but then it has taken the tree analogy too far by suggesting that different branches of hominins are ultimately connected to the main trunk – that of Homo sapiens. The tree of life however has no main trunk; in fact it has no trunk at all. The analogy of evolution with a tree is useful in the sense that it helps us conceptualize evolution as a branching process, where branches or species once sprung go their own way never to join together again. The concept of trunk is meaningless in evolution since there is no one main ancestor-descendent series in a branching evolutionary tree from which all other branches or species originate. Trunk is a retrospective label assigned to a particular ancestor-descendant series out of the many that may exist that we wish to highlight. The trunk metaphor is also suggestive of a belief in a ladder like progression of species, that a smooth tracing of ancestor and successive descendents in a chosen lineage defines the central inevitable pathway of progress in an evolutionary tree.

Ironically, this view of life describes the story of human evolution particularly poorly. The fossil record of Homo erectus (our immediate ancestor) indicates that populations of erectus were present over a vast area ranging from Africa to Europe to S. E Asia. These isolated populations of erectus diverged and evolved through independent ancestor-descendent successions in these various areas. We recognize some of these descendents by names such as the Neanderthals in Europe and W. Asia , Homo florensis in S.E Asia, while in others areas over Asia they are simply referred to as Homo erectus. Modern humans evolved from one such population of Homo erectus somewhere in East Africa. Scientists put the origin of the hominin lineage at around 5-7 million years ago of which the recent find is an early representative. Our species i.e. modern Homo sapiens appeared around 150,000 years ago. In between these two events several hominin species originated, prospered and went extinct. Our own species co-habited the planet with at least one other – perhaps more- hominin species. This evolutionary story brings to mind not a central trunk sprouting side branches of humanity and reaching its pinnacle in our origin but more a story of populations dispersed by migration seeding new lineages at places they settled, one of which led to us. Thus there was nothing inevitable about our origin. The feeling of uniqueness is an artifact of extinction; our evolutionary cousins are dead while we still live. This makes it seem as if evolution inevitably led to us. Such retrospective coronation is however faulty and one just has to look at lineages which are rich in living species to realize this. How for example from the profusion of modern species of beetles or flowering plants or rats will one identify one main ancestor-descendent succession or trunk? You can only describe these evolutionary trees as having many branches with many living representatives. Hominin evolution similarly did not consist of only one clear pathway, but many complex diverging ones.

The hominid Toumai, as has been rightly pointed out will re-energize the study of human evolution. However, it will not, contrary to what many have suggested, fundamentally change our views about how evolution works or change the basic story of humankind’s origins. The human evolution story, painstakingly pieced together from an impressive array of hominin fossils from Africa, Europe and Asia will also remain fundamentally unchanged. Toumai in fact, lends solid support to the most basic tenet of that story, which is our ancestors originated in Africa. It also gives a poke in the eye to evolution doubters who keep whining “show me the intermediates”, for here is a fossil with an impressive mix of primitive and derived features. Hopefully more fossils from that crucial time period of 5-7 m.y will be found in the near future. Those along with Toumai will help us better understand the details of our early evolution.

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