Universal common ancestry (UCA), also called universal common descent, is the theory, first proposed by Charles Darwin 150 years ago, that all creatures alive on Earth today are the descendents of one single-celled progenitor species. Douglas Theobald of Brandeis University recently confirmed this theory.
Although the huge amounts of genetic inter-relatedness between different species has lead to wide acceptance of the UCA theory, alternate theories were still possible. For example, it could have been that all multicellular organisms arose from one ancestor, and all microorganisms from another. More specifically, life could have arisen independently in each of the three main domains of life: Bacteria, Archaea, and Eukarya (including us).
To distinguish between these possibilities, Theobold compared the sequences of 23 essential proteins from organisms ranging from humans to flies to a variety of microorganisms. He was careful to use creatures from each of the three big domains listed above. All the tested organisms contained all 23 proteins, though with slight variations between them. He then asked whether the probability of finding that pattern of variation was most likely if the UCA model were true, or if one of the alternate models were true.
Based on his data, he concluded that the UCA theory was astronomically more probable than any multi-lineage theory. In other words, all life on Earth arose from a single progenitor species. In particular, the probability that humans did not descend from the same ancestor species as all other species on Earth was computed to be 1 in 10 to the 6000th power.
This does not mean that life cannot ever have arisen independently in Earth’s history. Life could have arisen multiple times, but only one version left descendents.
To me, this makes it seem more unlikely that there will be life forms on other planets. I know that it probably doesn't speak to that point (your last sentence, for example, gives a bit of hope that it isn't so very tough for life to get started)...but I really sorta-kinda hoped that some extreme life forms like bacteria found deep inside the earth, in really hot places, where there is no oxygen, would be unrelated. (Or deep-sea vent-loving critters, or ...well, you get the idea.)
ReplyDeleteThat's a really good point. I haven't seen the complete list of creatures Theobald used in his analysis. I suspect he used some 'extremophiles', but obviously not all of them. The fact that even deep sea tube worms and bacteria show similarities to other life forms on Earth certainly suggests that they will be found to fit the pattern.
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