Monday, April 12, 2010

Conceptualizing Evolution of A. Sediba And Homo With Diagrams

The recently named new hominid species from S. Africa Australopithecus sediba has generated a lot of articles and blog posts. Many of them are very informative and content rich. But going through them I found them to be text heavy and lacking in diagrammatic representations.

Its confusing trying to wade through all the possible scenarios of the relationship between Au. sediba and Homo that are being discussed. I found thinking of the relationships with diagrams eased some of that confusion.

So this post is relatively light on text and diagram rich. Here are evolutionary scenarios depicted in simple diagrams that I created:

1) Au. Africanus evolved into Au. sediba which then later evolved into Homo.

This scenario envisages evolution of a new species via phyletic gradualism in which the entire species transforms into a new species. Although new species could arise this way, in this case the fossils of Au. sediba are younger (1.7 - 1.9 mya) than the oldest representatives of Homo ( 2.3 mya) negating the possibility that Au. sediba transformed into Homo. John Hawks though outlines a situation where this model could still hold and that would be if the oldest representatives of Homo have been misclassified as Homo and are really australopithecines.

The next two scenarios are the ones favored by the authors of the paper.

2) Au. sediba is derived via cladogenesis from Au. africanus at an early date possibly as early as 3 million years or so.  Homo then evolves in a later speciation event from Au. sediba. The recently found fossils of Au. sediba represent a relict population. This is the punctuated equilibrium model of speciation wherein one population of a species becomes reproductively isolated from other populations and evolves into a new species. The ancestor and descendant species may then co-exist for a time period. 

3) Au. sediba and Homo share a common ancestor..either Au. africanus or a close cousin.

In both the above scenarios (2,3) the similarities between Au. sediba and Homo are due to a shared ancestry.

4) Many critics of the above interpretation prefer the following scenario.

Au. sediba evolved either via phyletic gradualism or via cladogenesis from Au. Africanus in S. Africa. Homo evolved from some other lineage of bipedal hominid elsewhere.  I have shown Au. afarensis as an ancestor of Homo but only as an example. There may have been several lineages of bipedal hominids and Homo could have been derived from within any one of them.

In this case the similarities between Au. sediba and Homo are a result of convergent evolution in response to similar locomotary and dietary pressures.

5) Donald Johanson, discoverer of Lucy favors this scenario.

Au. sediba has been misclassified as an australopithecine. It is really a representative of Homo. But Homo likely evolved somewhere in eastern Africa about 2.3 million years ago and the fossils found recently in S. Africa which are 1.7. to 1.9 my old is a migrant Homo population, descendants of east African Homo.

Plenty to chew on..

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