First Large-Scale Quantitative Test Validates Darwin's Theory of Universal Common Ancestry

More than 150 years ago, Darwin proposed the theory of universal common ancestry (UCA), linking all forms of life by a shared genetic heritage from single-celled microorganisms to humans. Until now, the theory that makes all living organisms distant relatives has remained beyond the range of a formal test. Researchers have reported the findings of the first large scale, quantitative test of the famous theory that underpins modern evolutionary biology.

The results of the study confirmed that Darwin was, in fact, correct. In his 1859 book, On the Origin of Species, the British naturalist proposed that, "all the organic beings which have ever lived on this earth have descended from some one primordial form.” Over the last 150 years, qualitative evidence for this theory has grown, in the numerous, surprising transitional forms found in the fossil record, for example, and in the identification of far-reaching essential biologic similarities at the molecular level.

Still, discourse among some evolutionary biologists have recently emerged questioning whether the evolutionary relationships among living organisms are best described by a single "family tree” or rather by multiple, interconnected trees--a "web of life.” Recent molecular evidence indicates that primordial life may have undergone rampant horizontal gene transfer, which occurs frequently today when single-celled organisms swap genes using mechanisms other than usual organismal reproduction. In that case, some scientists argue, early evolutionary relationships were web-like, making it possible that life sprang up independently from many ancestors.

According to biochemist Dr. Douglas Theobald, from Brandeis University (Waltham, MA, USA), it does not really matter. "Let's say life originated independently multiple times, which UCA allows is possible,” said Dr. Theobald. "If so, the theory holds that a bottleneck occurred in evolution, with descendants of only one of the independent origins surviving until the present. Alternatively, separate populations could have merged, by exchanging enough genes over time to become a single species that eventually was ancestral to us all. Either way, all of life would still be genetically related."

Harnessing powerful computational tools and applying Bayesian statistics, Dr. Theobald found that the evidence overwhelmingly supports UCA, regardless of horizontal gene transfer or multiple origins of life. Dr. Theobald said UCA is millions of times more probable than any theory of multiple independent ancestries. "There have been major advances in biology over the last decade, with our ability to test Darwin's theory in a way never before possible,” said Dr. Theobald. "The number of genetic sequences of individual organisms doubles every three years, and our computational power is much stronger now than it was even a few years ago.”

Whereas other scientists have previously examined common ancestry more narrowly, for example, among only vertebrates, Dr. Theobald is the first to test formally Darwin's theory across all three domains of life. The three domains include diverse life forms such as the Eukarya, as well as Bacteria and Archaea.

Dr. Theobald examined a set of 23 universally conserved, basic proteins found in all known organisms. He chose to study four representative organisms from each of the three domains of life. For example, he researched the genetic links found among these proteins in archaeal microorganisms that generate marsh gas and methane in cows and the human gut; in fruit flies, humans, round worms, and baker's yeast; and in bacteria such as Escherichia coli and the pathogen that causes tuberculosis.

Dr. Theobald's study rests on several simple suppositions about how the diversity of modern proteins arose. First, he assumed that genetic copies of a protein could be multiplied during reproduction, such as when one parent gives a copy of one of their genes to several of their children. Second, he assumed that a process of replication and mutation over the eons might modify these proteins from their ancestral versions. These two factors, then, should have created the differences in the modern versions of these proteins seen throughout life today. Lastly, he assumed that genetic changes in one species do not affect mutations in another species--for example, genetic mutations in kangaroos do not affect those in humans.

What Dr. Theobald did not hypothesize, however, was how far back these processes go in linking organisms genealogically. These processes are able to link the shared proteins found in all humans to each other genetically. However, the answers to if the processes in these assumptions link humans to other animals; if these processes link animals to other eukaryotes; and if these processes link eukaryotes to the other domains of life, bacteria, and archaea turns out to be a definitive yes.

Just what did this universal common ancestor look like and where did it live? Dr. Theobald's study does not answer this question. Nevertheless, he speculated, "to us, it would most likely look like some sort of froth, perhaps living at the edge of the ocean, or deep in the ocean on a geothermal vent. At the molecular level, I'm sure it would have looked as complex and beautiful as modern life.”

The study's findings were published in the May 13, 2010, issue of the journal Nature.

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