Is the tree of life really a ring?
October 1, 2010 Leave a comment
The tree of life is really a ring
When Darwin proposed his ideas about how new species arise, he produced a metaphor that we still adhere to today to explain the branching patterns of speciation: The Tree of Life. This metaphor for the way one species may branch from another through changes in allele frequencies over time is so powerful and of such long standing that many large studies of the speciation process and of life’s origins carry its name.
It may be time for a name change. In 2004, an astrobiologist and molecular biologist from UCLA found that a ring metaphor may better describe the advent of earliest eukaryotes. Astrobiologists study the origins of life on our planet because of the potential links between these earthly findings and life on other planets. Molecular biologists can be involved in studying the evolutionary patterns and relationships that our molecules—such as DNA or proteins—reveal. Molecular biologist James Lake and astrobiologist Mary Rivera of UCLA teamed up to examine how genomic studies might reveal some clues about the origins of eukaryotes on Earth.
Vertical transfer is so 20th century
We’ve heard of the tree of life, in which one organism begets another, passing on its genes in a vertical fashion, with this vertical transfer of genes producing a tree, with each new production becoming a new branch. The method of gene transfer that would produce a genuine circle, or ring, is horizontal transfer, in which two organisms fuse genomes to produce a new organism. The ends of the branches in this scenario fuse together via their genomes to close the circle. It is this fusion of two genomes that may have produced the eukaryotes.
Here, have some genes
Eukaryotes are cells with true nuclei, like the cells of our bodies. The simplest eukaryotes are the single-celled variety, like yeasts. Before eukaryotes arose, single-celled organisms without nuclei—called prokaryotes—ruled the Earth. We lumped them together in a single kingdom until comparatively recently, when taxonomists broke them into two separate domains, the Archaebacteria and the Eubacteria, with the eukaryotes making up a third. Archaebacteria are prokaryotes with a penchant for difficult living conditions, such as boiling-hot water. Eubacteria include today’s familiar representatives, Escherichia coli.
According to the findings of Lake and Rivera, the two prokaryotic domains may have fused genomes to produce the first representatives of the Eukarya domain. By analyzing complex algorithms of genomic relationships among 30 organisms—hailing from each of the three domains—Lake and Rivera produced various family “trees” of life on Earth, and found that the “trees” with the highest cumulative probabilities of having actually occurred really joined in a ring, or a fusion of two prokaryotic branches to form the eukaryotes. Recent research If we did that, the equivalent would be something like walking up to a grizzly bear and hand over some of your genes for it to incorporate. Being eukaryotes, that’s not something we do.
Our bacterial parentage: the union of Archaea and Eubacteria
Although not everyone buys into the “ring of life” concept, their findings help resolve some confusion over the origins of eukaryotes. When we first began analyzing the relationship of nucleated cells to prokaryotes, we identified a number of genes—that we call “informational” genes—that seemed to be directly inherited from the Archaea branch of the Tree of Life. Informational genes are involved in the processes like transcription and translation, and indeed, recent “ring of life” research suggests a greater role for Archaea. But we also found that many eukaryotic genes traced back to the Eubacteria domain, and that these genes were more organizational in nature, being involved in cell metabolism or lipid synthesis.
Applying the tree metaphor did not help resolve this confusion. If eukaryotes vertically inherited these genes from their prokaryotic ancestors, we would expect to see only genes representative of one domain or the other in eukaryotes. But we see both domains represented in the genes, and the best explanation is that organisms from each domain fused entire genomes—horizontally transferring genes—to produce a brand new organism, the progenitor of all eukaryotes: yeasts, trees, giraffes, killer whales, mice, … and us.