How did animals evolve? – A matter of perspective

Jellyfish - acobox.com

Isn’t it funny how nobody ever approaches the same problem in quite the same way? Personally, I am always fascinated by how different angles of attack on a scientific problem can lead to different solutions. One example of this phenomenon that I encountered recently concerns the evolution of the first animals.

As animals ourselves, it’s unsurprising that a fair amount of scientific resources are directed towards studying their evolution. But despite all this grant money, we still don’t know:

1. When did they first appear?
2. What did they evolve from
3. Which, if any, of the multitude of Precambrian fossils represent metazoans?

A review of the recent scientific literature suggests that the date for their first appearance is gradually drifting towards 700-500 Ma. The uncertainty is largely due to the discrepancy between molecular clock dates (which suggest ages as far back as 1 Ga) and the fossil record.

The Metazoan Fossil Record

Ediacaran macrofossil - Source: awildsheepchase on Flickr

The Precambrian fossil record is patchy at best and always ready to pounce on the unwary. Last year El Albani et al. (1) published a paper in Nature suggesting that some large pyritised squiggles were 2.1 Ga metazoans. Unfortunately they are almost certainly just large pyritised squiggles with no biogenic relationship, as a Google Image search for “pyrite concretions” will suggest.

Fast forward 1.5 billion years and we get to the Ediacaran period. This has no shortage of large macroscopic organisms. But are any of them recognisably, unambiguously animalian?

Sure, there’s Kimberella, the putative mollusc, and no shortage of “vendobionts” (whatever they may be) but bona fide animals are few and far between. Not until we hit the Cambrian do we suddenly get a (relatively) rapid “explosion” of animals.

“Explosion”?

Sceptical gentleman like myself tend to recoil from terms like “explosion” in the context of evolution. Empty ecospace or not, the paucity of the fossil record is always a concern when studying macroevolutionary trends.

Besides, if evolution as we know it is true, then there must be a stem lineage lurking about somewhere.

Could it be the Ediacaran fauna: the peculiar Charnia, the weird Dickinsonia and their fascinating relatives? More intelligent people than me (a large category) have suggested so (2).

This is all well and good but for the purpose of this essay it is somewhat irrelevant.

More interesting are the recent suggestions of Nick Butterfield (e.g. 3) that the appearance of heterotrophic metazoan zooplankton in the oceans at this time caused caused large scale ecological and biogeochemical reorganisation.

Environment vs. Biology

This is an interesting hypothesis, not least because there are no shortage of authors who have suggested the opposite: that the rise of oxygen enabled the animals (the “permissive environment” hypothesis).

Perhaps one of the most influential of these is Don Canfield, a geochemist. Butterfield, by contrast, is a palaeontologist.

So we have an interesting situation where the biologist argues for the importance of biology and the chemist sticks up for chemistry. Hardly surprising when you think about it.

Middle ground?

Any historian of science, particularly biology, cannot fail to notice that the majority of polemic disputes end with theory accounting for both end-member scenarios. Canfield’s recent papers suggest that Neoproterozoic oxygenation started some hundred million years before the onset of the Ediacaran and the first evolutionary signs of metazoans (e.g. increased evolutionary turnover, macro-organisms). Conversely, Butterfield presents a fairly convincing case for the impact of life on the evolution of the planet.

The progressive co-evolution of both seems a reasonably solution to this problem although it doesn’t entirely limit the fruitless chicken-or-the-egg debate. However, we don’t currently know the timings or details of the environmental or evolutionary changes accurately enough to distinguish between these three hypotheses.

I am also aware that I am making sweeping generalisations here – there are no shortage of geochemists/palaeontologists who favour the other explanation.

The message I am trying to get across is that yet again, it pays to ask: “What are the biases of the person telling me this stuff?”

1. El Albani et al. (2010) Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago; Nature
2. Xiao and Laflamme (2008) On the eve of animal radiation: phylogeny, ecology and evolution of the Ediacara biota Trends in Ecology and Evolution
3. Butterfield (2011) Animals and the invention of the Phanerozoic Earth system Trends in Ecology and Evolution