David Tyler: Tetrapod family tree looks like a bush

The transition from fish to land animal is regarded by many as well documented: it is number two in Nature's presentation of "15 evolutionary gems". Some of the names given to members of the tetrapod lineage are quite well known: Panderichthys and Tiktaalik, Ichthyostega and Acanthostega, for example.

Open any paleontology text or children's book on prehistoric animals, and you will find something between fish and tetrapod, forelimbs or fins planted on the land, tail receding into the water, eyes cast hopefully forward. These images encapsulate an episode of vertebrate history spanning the latter half (390 to 360 million years ago) of the Devonian, the waning days of the "Age of Fishes."

Heartwarming. I loved those stories. However,

Earlier in the fossil record, there are fish and no tetrapods; later in the fossil record, there are clear signs of terrestrial tetrapods - so, it is inferred, there has to be a transition between them. The more we now of the fossil record, however, the more difficult it is to identify an evolutionary branch. Instead of a tree, we see a bush. But this means that the quest for missing links will be elusive. We cannot identify ancestors and descendants in the data accessible to us. This changes the nature of the debate.

It sure does. The tree of life of Darwinian evolution is probably a legend, one that may not even compare favourably with the Tree of Life in the Bible, if you go by its significance in understanding human life and behaviour.

One big problem is, no one sees the transitions in progress, so we do not really know for sure whether they go fast or slow or what drives them. Maybe the story is slightly different in each case.

The journal article discussed is: Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs, Viviane Callier, Jennifer A. Clack, and Per E. Ahlberg, Science, 324 17 April 2009: 364-367 DOI: 10.1126/science.1167542

For more on the tree of life, go here

For more "tree/bush.groundcover of life" stories from the Post-Darwinist, go here. Especially,

- Darwin's Tree of Life splinters (podcast)

Will gene-swapping fell the prokaryote tree of life?

- Tree of life for plants now a ground cover

Tree of life (Tyler) Bush or forest of life better explanation? Alternatives to common ancestry

Tree of life (Tyler) - force fitting explanations to defend an orthodoxy

Tree of life (Tyler) as unnecessary concept that cannot be justified by empirical data

(Note: The image is of the tree of life is from Wikimedia Commons.)