Friday, November 30, 2007

Evolution: Digging up the past

The book Digging Up The Past (Paul Willis and Abbie Thomas) is a survey of Australia's chief paleontological sites, with a bit of history thrown in. For this, it is a very worthwhile book; the following thoughts emerge.

1) Digging for fossils is not an enviable occupation, despite the glorifying fiction.
Mostly, it seems to largely amount to volunteers laboriously toiling in unpleasant conditions (in Australia at least), only to have someone else identify and name the results of your ardure.

2) How did Australia get so dry? It's not what you think.
Australia once had much more forest and grassland. But it moved north, separating from Antarctica and South America. New Guinea appeared from the sea, and rose - whether through tectonics or volcanic activity. Willis/Thomas state that the very appearance of NG prevented the ameliorating monsoonal rains from the north to penetrate with the necessary strength and frequency.

3) Convergent evolution is an ongoing debate.
Although descriptive passages are probably intended to evoke rather than draw genetic equivalences, Willis/Thomas suggest quite a lot of convergent evolution of types of marsupials that have [been described as having] strong resemblances to placentals elsewhere.
a) There's pause for thought on similar environmental niches [with a macro consistency] can mould common directions. But it must be largely dependent on the stage of development of plant life (which is an environmental determinant for animals), not to say the stage of development of the planet as a whole, including (say) oxygen levels, but excluding cyclical variables such as temperature, ocean levels, etc.
b) the flourishing of mammals (and within that, placentals vs marsupials) may be seen to mirror the great diversification of dinosaurs, albeit over a shorter time.
In other words, there seems to be much pattern-forming, and similarities within those patterns.
c) Humans have fully disrupted this progression/cycle mix - for the entire length of their stay on this planet, at least. One can easily speculate that, were humans to disappear - after having caused this major extinction event - yet another life form would diversify over the planet.

4) The study of marsupials can provide some particular insights into evolution overall, particularly vis-a-vis placentals.

5) Consider the apparently ungainly tree kangaroo (p286). Its ancestor is described as being more so. But again, that is only in our eyes, and all this is part of the process of adaption. For any given environmental niche, over the time that that niche was stable, the extant species would have been ipso facto successful.

6) Herbivore size (p289): The rule of thumb given is that the sparser the food, the larger the species needs to be to eke out survival.

These are just some thoughts arising from the book; this is not intended to be a review. The value in the book lies in the collation of Australian sites of paleontological significance, and the revealing of Australia as a key destination for an understanding of the history of evolution.

Willis, Paul and Thomas, Abbie (2004): Digging Up Deep Time, ABC, Sydney.

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