Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Thursday, April 07, 2005

Yet another missing link

New evidence supports the claim to being a hominid of the Toumai skull found in Chad, which gave its name to Sahelanthropus tchadensis. The find is regarded by some as having a "a puzzling combination of human and chimp traits".

Transitional forms between two modern groups are sometimes called "missing links", but this is a misnomer based on the pre-Darwinian idea that there should be an unbroken series of gradual changes over time from simple organism to modern organism. A "missing link" would be an unfound member of that series, which is like a morphing graphic with a missing frame.

But species, while they have a history from their ancestors, do not fill in all the possible intermediates. Speciation involves greater or lesser modification, and the sequence is not a simple consistent morph. It is thought that much modification in speciation occurs early on, and the species tends to remain stable for the remainder of its "lifespan". While the modification is gradual in one way (there will not be massive adaptive change in a single or few steps, and so from the point of view of a human observer, change would be extremely gradual. But evolution happens on geological, not sociological, time scales), in terms of what will be preserved, it will be episodic.

The type of "transitional form" here is of a species that probably evolved in its own way such that we think it is like both groups: apes and hominids. It is a twig between the two main branches. But they only became main branches after the fact. At the time, tchadensis was probably just another ape species, one of many, like the ancestor of modern hominids (i.e., us). Neither the African Great Apes nor Hominids were at that time a main branch.

A lot of the problem people have with conceptualizing evolution is that so much of what seems "significant" or "different" is assignable only after the fact. When evolution is happening, it is not aiming to deliver Apekind and Humankind. It just delivers apes that survive, or not, leaving greater or less record in the living and preserved organisms we have found.

When we are growing up and learning language, the words apply to "obviously different" groups. But from the perspective of the evolutionary tree, they are not so different, and other things we think are less different, like the many species of bats or ants that we naively group into a single "kind", are recognizable as massively diverse. Bats form 1100 or so species, out of the total of around 4000 species. 1 in 4 mammal species is a bat species. But most of the time, we are happy to say "It's a bat!"

Diversity is not a subjective fact about biology. It's real. So if we do biology, rather than teach third grade children the meanings of ordinary words like "bat" or "cat" (36 species, including 9 tiger species), then we need to pay attention. Evolution explains why the diversity is there. It also explains why it has the distribution over space and time it does. We find the transitionals we expect to find, like tchadensis. But we don't find "missing links". That's just journalistic hyperbole. Ignore the headlines and look at the information. It's much more interesting.