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THE tree of life needs new roots, according to a controversial new theory of
how our earliest single-celled ancestors were related to each other. The theory
dismisses the notion that life first evolved in hot, extreme environments. It
suggests instead that our oldest ancestors were cool-living, sun-worshipping
bacteria.

Biologist Tom Cavalier-Smith of Oxford University set out to explain how the
first primitive cells seeded the complex array of life we see today. Biologists
divide life into two key groups: the prokaryotes, which don鈥檛 have their DNA
enclosed in a membrane-bound nucleus; and eukaryotes, which do.

The eukaryotes include yeasts, plants and all multicellular animals,
including ourselves. Prokaryotes include eubacteria鈥攖he 鈥渃ommon or garden鈥�
bacteria such as Salmonella and E. coli鈥攁nd the
mysterious archaebacteria that seem to have a mix of bacterial and eukaryotic
features.

Archaebacteria were only recognised as a distinct form of life in the late
1970s. They often live in extreme environments such as hot springs and
hydrothermal vents, leading many scientists to conclude that they might have
evolved in the severe environment of early Earth.

The idea is borne out by studies comparing small differences in genes
encoding a molecule called ribosomal RNA. These indicated that archaebacteria
branched off from very simple cells over 3 billion years ago, which puts them
near the roots of the evolutionary tree. But Cavalier-Smith says that this
widely accepted 鈥渕olecular clock鈥� technique is unreliable. He also thinks
studies of single molecules are too simplistic. 鈥淐ells and organisms are
composed of interdependent parts that never evolve entirely independently,鈥� he
points out. So he adopted an approach that takes this into account.

By comparing the detailed cell biology of archaebacteria, eubacteria and
eukaryotes, and studying the fossil record, he claims to have shown that the
archaebacteria aren鈥檛 nearly as ancient as we think. In fact, he says they are
close cousins of eukaryotes that probably first evolved a mere 850 million years
ago in the late Precambrian, and that both are descended from the eubacteria
that later gave rise to TB and leprosy.

The suggestion that archaebacteria and eukaryotes are descended from
eubacteria turns established thinking on its head, yet it鈥檚 also plausible, says
Andrew Roger, a molecular evolutionist at Dalhousie University in Nova Scotia.
It means that eubacteria have been around four times as long as archaebacteria,
and were the first forms of life. Cavalier-Smith thinks the oldest cell was
probably a green, photosynthetic bacterium that didn鈥檛 take kindly to a hot
environment.

鈥淭his is a radical departure from what has previously been believed,鈥� agrees
Alex Jeffries at the University of Bath. He points out that there is geological
evidence that something was producing methane billions of years ago, and
archaebacteria are the only bugs thought to be able to do this. But
Cavalier-Smith counters that archaebacteria aren鈥檛 found in the fossil record
from more than 850 million years ago.