|17 May 2010
Universal common ancestor theory upheld by new study
A broad-based, statistical study of genes from all three domains of life on Earth has found Darwin's UCA theory to be millions of times more probable than the competing hypothesis of multiple independent origins.
Douglas Theobald, a professor of biochemical evolution at Brandeis University in Massachusettes, U.S.A., examined DNA from Eukarya, Bacteria and Archaea. Studying 23 essential proteins found in all organisms, he discovered that these proteins link eukaryots to bacteria, bacteria to archaeans, and archaeans to eukaryots. Previous work had been limited to one domain. His results were published in the journal, Nature.
Eukarya are organisms whose cells have a nucleus, and include humans and other animals and plants. Bacteria and Archaea are both procaryotes, but organisms in the two domains differ genetically from each other as much as they do from Eukarya. Archaea are the species found in environments on Earth that would be extreme for other lifeforms. They inhabit hot springs and deep sea rift vents and can survive in highly acidic or alkaline conditions.
Darwin published his UCA theory in 1859 and it was supported by evidence from the fossil record. As biologists began to use DNA to map evolution, they discovered evidence of horizontal gene transfer. In this process, microorganisms exchange genetic material apart from the normal reproductive cycles. Based upon these data, an alternate 'web of life' hypothesis was developed allowing for multiple independent origins.
Theobald argues that this process is not inconsistent with the Universal Common Ancestor idea. UCA would require that at some point in evolutionary history, the various lifeforms merged or that decendents of only one species survived to the present. Theobald's results support this idea, but cannot determine which occurred.
The study was based upon assumptions about the origin of diversity in modern proteins. Protiens are replicated during reproduction when parents pass genetic material to offspring. During that process, variations and mutations can occur, causing subtle changes in structure and function. These modified versions will then be passed to a new generation. Theobald also assumed that mutations in one species will not affect another if there is no exchange of genetic material.
No portrait of our common ancestor can be provided by the work that was done. Theobald speculates that we evolved from "some sort of froth, perhaps living at the edge of the ocean, or deep in the ocean on a geothermal vent. I'm sure it would have looked as complex and beautiful as modern life."
Source: EDP 24 (12 May 2010)
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