LUCA and the thicket of life

Why is universal common descent so important, though? What does it actually do? It affirms Charles Darwin, of course, who famously wrote of life being “breathed into few forms or one”, but his theory didn’t actually demand it scientifically. He wrote against a prevailing assumption that natural species – if not artificially selected varieties – were unchanged since creation, but descent with modification needn’t imply a single ancestor, and the fossil record available to  him certainly didn’t support that.

In fact, if as he thought evolution were a product of natural law, one would expect multiple origins. Life should be easily repeatable. A single ancestor implies a highly contingent event with a special cause – a contradiction still valid today. But as much research including a recent study confirms,  LUCA was already a complex organism which included organelles, suggesting that today’s simpler lifeforms like bacteria have devolved, rather than eukaryotes growing in complexity. The paper does not, of course, suggest that LUCA was created de novo. Theobald’s 2010 study, which showed a single ancestor to be far more probable than multiple origins, did not actually claim that life arose only once:

Theobald’s study does not address how many times life may have arisen on Earth. Life could have originated many times, but the study suggests that only one of those primordial events yielded the array of organisms living today. “It doesn’t tell you where the deep ancestor was,” Penny says. “But what it does say is that there was one common ancestor among all those little beasties.”

In other words, multiple original ancestors could have evolved and exchanged genes over hundreds of millions of years, but only one form avoided eventual extinction to give rise to today’s life. So LUCA is as much a black box to modern life as mitochondrial Eve is to humans: it tells us about historical contingency in the form of a bottleneck, but not about origins.

But now another study  adds weight and substance to the hypothesis that viruses devolved from complex cells. It was already known that large amounts of viral genetic material have been introduced into the genomes of complex organisms, one of the most potent and current forms of gene transfer. Whilst that material was restricted to the simple components of RNA viruses and so on, it might be argued whether transposons derived from viruses or vice versa, but at least the results could be distinguished from coding DNA.

But if giant viruses containing the genes of their previous cellular life are, or were, more common than we assume, HGT might be far more extensive in post-LUCA evolution than we know. Maybe it could even account for many of those instances where phylogenetic and taxonomic relationships differ. Or to put it another way, we may be as unable to trace common ancestry with confidence in the lifeforms we know as we are in the precursors of LUCA. Perhaps the most we can really say is that all life is related, on the basic principles of a shared genetic code, body chemistry and so on. That’s a long way from common ancestry, and even further from making common ancestry an incontrovertible biological axiom.

An analogy would be the human MRCA studies I have cited elsewhere to show the plausibility of a historical Adam in historical time. The fact that we can show mathematically that the current human race had a common ancestor within a few thousand years of the present doesn’t actually tell us much about the descent of the human species. It might be relevant to theology, and vaguely interesting to historical anthropology. But in terms of saying where Homo sapiens came from, so what?

If there’s any truth in my suggestion, then the problem is that the neat Darwinian tree of life does not become a bush, or even a forest, but a tangled and impenetrable thicket. How does one make a simple and persuasive pattern out of that?

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About Jon Garvey

Training in medicine (which was my career), social psychology and theology. Interests in most things, but especially the science-faith interface. The rest of my time, though, is spent writing, playing and recording music.
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