THE trunk of the tree of life鈥攖he so-called 鈥渦niversal ancestor鈥� from
which all later life forms branched鈥攎ay be a tangled thicket instead of a
single stem, says an American evolutionary biologist. If true, this would
dramatically change the way biologists view the early history of life on
Earth.
Biologists have assumed that if they learnt enough about evolutionary
history, they could trace the evolutionary tree from one ancestor to another all
the way back to the first living cells. Most believe the earliest organisms were
much like bacteria or archaeans鈥攕ingle-celled organisms similar to
bacteria鈥攐r had characteristics of both.
But this genealogy breaks down if you follow it back more than 3 billion
years, says Carl Woese of the University of Illinois, Urbana-Champaign. 鈥淭he
phylogenetic tree does not have its root in anything we would call an organism
by today鈥檚 standards,鈥� he says. The reason for this breakdown is a process
called lateral gene transfer, in which organisms acquire genes not from their
parents but by picking up stray fragments of DNA from organisms in their
environment.
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There is ample evidence that lateral transfers took place early on. For
example, enzymes that join up amino acids to make proteins, aminoacyl-tRNA
synthetases, show a confused pattern of relatedness among the three main
branches of the tree of life鈥攁rchaeans, bacteria and the more complex
eukaryotes that contain nuclei.
Some synthetases, such as that for isoleucine, are most similar between
archaeans and eukaryotes鈥攖he two branches of the tree that other evidence
suggests are most closely related. But Woese has found that others, such as that
for valine, are most similar between bacteria and eukaryotes. Lateral transfer
of the gene for the valine enzyme between bacteria and eukaryotes could explain
this discrepancy. Likewise, there are odd affinities between bacteria and
archaeans that suggest lateral transfer.
Such transfers are rare today because evolution has fine-tuned modern
organisms to the point that their parts are no longer interchangeable. Woese
hypothesises that among the crude cells of the earliest life forms, lateral gene
transfer could have been much more common (Proceedings of the National
Academy of Sciences, vol 95, p 6854).
This genetic fluidity, Woese says, may have been so extensive that the
universal ancestor of modern life forms was not a single organism but a loose
community of protocells that swapped genetic material so frequently that they
evolved together. The three main branches of life probably emerged as subsets of
this community, whose genetic makeup gradually became more rigid and less open
to lateral transfer as evolution favoured sets of genes that had become adapted
to one another.
If Woese is right, researchers may need to rethink the history of life.
Different parts of an organism鈥檚 genome may have different evolutionary
histories. 鈥淎t some level, we should be looking at the evolution of genes, not
genomes,鈥� says Ford Doolittle, a biologist at Dalhousie University in Halifax,
Nova Scotia.