Friday, 13 September 2013

Evolution's 'Big Bang' Explained

Marine life during the Cambrian explosion (~520 million years ago).
Image credit: Katrina Kenny & Nobumichi Tamura.
Copyright © 2010 The University of Adelaide
Biologists measure evolution's Big Bang

Unlike religion, where problems are disturbing and need special measures to explain them away or suppress them and keep knowledge of them away from the faithful, science thrives on them and actively looks for them.

One such problem has for a long time been the so-called 'Cambrian Explosion' in which a large number of very different major taxa suddenly seem to appear in the fossil record over a relatively short time scale, suggesting a rate of evolution which is hard to explain in conventional Darwinian terms. However a paper published today in Current Biology by Associate Professor Michael Lee of Adelaide University, Australia, and Dr Greg Edgecombe of the Natural History Museum of South Australia, to which the above link relates, shows that, although there was indeed rapid evolution immediately prior to and into the Cambrian Era, it was not inconsistent with Darwinian theory.

First a little background:

The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 542 million years ago, of most major animal phyla, as demonstrated in the fossil record. This was accompanied by major diversification of other organisms. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude and the diversity of life began to resemble that of today. All present phyla appeared within the first 20 million years of the period, with the exception of Bryozoa, which made its earliest known appearance in the upper Cambrian.

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.


Incidentally, note that Darwin mentioned this problem and highlighted it as one of the main objections to his (and Wallace's) theory in line with my opening sentence. Darwin was honest and open-minded enough to admit that his theory was not without problems and even to point out its areas of weakness to his detractors. Try asking your local priest, imam or rabbi, or one of today's professional apologists for religion, which their theories' major areas of weakness are, and see how their honesty compares to that of Darwin.


A living arthropod (centipede Cormocephalus) crawls over its 515-million-year-old
relative which lived during the Cambrian explosion (trilobite Estaingia).
Copyright © 2010 The University of Adelaide
The Cambrian Explosion was the subject of a beautiful 1989 book, Wonderful Life, by the late Stephen Jay Gould in which he dealt with the Cambrian fossils of the Burgess Shales as re-analysed by Harry B. Whittington and colleagues. Gould's conclusion was that there must be some other explanation for these apparent bursts of rapid, almost sudden, evolution which he termed 'Punctuated Equilibrium', a hypothesis which he developed with Niles Eldredge and which led to the then famous rift between Richard Dawkins and Stephen Jay Gould. This rift was elevated to the status of warfare by creationists and was itself the subject of a book by Kim Sterelny, Dawkins Vs Gould: Survival of the Fittest, but which Dawkins characterised as a storm in a teacup; a mild disagreement by two Darwinists over the precise details.

In the 1990s most American evolutionists argued PE while most of the rest of the world remained traditionally Darwinian. Some of us saw the spectre of American nationalism in this and interpreted Gould's motives as an attempt to cash in on it by attempting to redefine the theory of evolution and update it with a new improved all-American version. We saw parallels between this and the jingoism which led many English palaeoanthropologists to fall for the Piltdown hoax because their nationalism led them to assume that, if God was going to evolve humans from monkeys, then, because English people are obviously the most highly evolved humans, Southern England is exactly where he would have chosen to do it. Nationalism is rarely appropriate, and often harmful, in science.

Meanwhile the US-led creationists lapped up this inter-tribal warfare as 'proof' (how they love that word!) that science is useless for discovering the truth when even the top people can't agree. Anyway, it would need a god (the locally popular one, naturally) to make PE work, so Gould, the Burgess shales and Punctuated Equilibrium had settled the matter - God did it!. How to explain all that rapid evolution since Noah? Easy! Punctuated Equilibrium explains it and the Cambrian Explosion showed it had happened before.

And it had taken an American to prove it, unlike that godless English Darwin, even if the American in question didn't realise it, and even if he could never come up with a convincing biological explanation for how this might work, other than by Darwinian natural selection acting on small variations in the population in an unguided, undirected, aimless process.

And now all that was academic. If today's paper is correct, Darwin (and Dawkins) were right. The 'Cambrian Explosion', even if it's not just the illusory artefact of an incomplete fossil record, is not a major problem for 'one-small-step-at-a-time' Darwinian evolution. It was fast, but not that fast.

Summary
The near-simultaneous appearance of most modern animal body plans (phyla) ~530 million years ago during the Cambrian explosion is strong evidence for a brief interval of rapid phenotypic and genetic innovation, yet the exact speed and nature of this grand adaptive radiation remain debated. Crucially, rates of morphological evolution in the past (i.e., in ancestral lineages) can be inferred from phenotypic differences among living organisms—just as molecular evolutionary rates in ancestral lineages can be inferred from genetic divergences. We here employed Bayesian and maximum likelihood phylogenetic clock methods on an extensive anatomical and genomic data set for arthropods, the most diverse phylum in the Cambrian and today. Assuming an Ediacaran origin for arthropods, phenotypic evolution was ~4 times faster, and molecular evolution ~5.5 times faster, during the Cambrian explosion compared to all subsequent parts of the Phanerozoic. These rapid evolutionary rates are robust to assumptions about the precise age of arthropods. Surprisingly, these fast early rates do not change substantially even if the radiation of arthropods is compressed entirely into the Cambrian (~542 mega-annum [Ma]) or telescoped into the Cryogenian (~650 Ma). The fastest inferred rates are still consistent with evolution by natural selection and with data from living organisms, potentially resolving "Darwin’s dilemma." However, evolution during the Cambrian explosion was unusual (compared to the subsequent Phanerozoic) in that fast rates were present across many lineages.


My feeling all along has been that periods of relatively rapid evolution, interspersed with periods of relative stasis are what we should expect of Darwinian evolution, given what we know of the radiation of different clades and species when a new niche is opened up by development of some feature like photosynthesis, flight, lungs, limbs, etc. Founder effects and genetic drift in a relatively non-competitive environment are enough to explain it. I would go so far as to suggest that it would be harder to explain evolution progressing at a completely steady rate in these circumstances.

In the early days (rather millions of years) of the evolution of life on Earth, especially that of eukaryote evolution from prokaryote cells and then again that of multicellularity from single-celled prokayrote cells, we should expect to see this radiation with so many previously unexplored potential niches being available. At the body-plan development stage we should expect this radiation to consist of 'experiments' in basic body-plans, with competition between different plans eventually leading to extinctions of some and success for others as the niches fill and competition increases.

We should also expect there to be a significant environmental impact as living things moved into new areas and interacted with them. These changes would be expected to feed back into the evolutionary process with natural selection favouring some plans and not others in a reinforcing process which would accelerate evolution. This is exactly why we see the evidence of radiation throughout nature today and probably why we see evidence of rapid radiation of multicellular body-plans in the Cambrian.

Like the cosmological Big Bang, the evolutionary 'Big Bang' is not a problem for science; it's a problem for religious superstition.

Reference:
Lee et al., Rates of Phenotypic and Genomic Evolution during the Cambrian Explosion, Current Biology (2013), http://dx.doi.org/10.1016/j.cub.2013.07.055





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1 comment :

  1. as you pointed out. The biggest problem for creationists with Cambrian explosion is their misinterpretation of the word explosion (fast) which actually took 50 million years (slow).

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