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THE distant ancestor of complex life forms, including humans, came into being
when a virus infected a simple bacterium-like cell and took control, according
to a controversial new theory. But many experts are finding it hard to accept
the idea that simple viruses could have had such a big impact on evolution.

Philip Bell of Macquarie University in Sydney devised his theory to explain
how the cells that make up complex creatures such as humans got their nuclei,
the control centre that contains most of the cell’s genes. Biologists accept
that other parts of complex cells, such as mitochondria, are the remnants of
bacteria that infected a simpler type of cell. But most believe that viruses are
no more than packages of genes that started out as parts of complex cells and
then went freelance, rather than the other way round.

“It’s a refreshing idea I never expected to see,” says molecular evolutionist
Peter Gogarten of the University of Connecticut in Storrs. “I don’t know if I
believe it, but it is definitely worth thinking about critically.”

Explaining how complex plant and animal cells evolved from the two known
types of simple cells, bacteria and archaea, is tricky. One popular theory is
that the ancestor cell was created when bacteria and archaea merged. That
explains why complex cells have some features in common with both archaea and
bacteria. But it does not account for many of the differences.

For instance, the genes of complex cells, or eukaryotes, are packaged up in
linear rod-shaped chromosomes inside the nucleus. Most bacteria and archaea
(together called prokaryotes) have circular chromosomes that float freely within
the cell. Complex cells also cap their RNA molecules with noncoding sequences,
while prokaryotes do not.

Bell’s theory is based on the fact that some viruses, including a group
called pox viruses, have linear DNA like we do. Pox viruses also share other
features with the nuclei of complex cells, like the RNA caps. Bell has backed up
his idea by comparing the sequence of an enzyme involved in putting on the caps
both in eukaryotes and in viruses. His results show that the enzyme probably
moved from the virus to the cell, rather than the other way round.

Bell also points out that unlike most simple cells, viruses and complex cells
can fuse their membranes—this is what happens when a sperm and egg join
together. Bell proposes that viruses passed on this ability to the eukaryote
ancestor, and that it enabled the cell to become compartmentalised and for
sexual reproduction to evolve.

But so far Bell has failed to convince many experts. According to Hyman
Hartman of the Massachusetts Institute of Technology in Cambridge, the theory
doesn’t explain enough of the novel features of complex cells, such as how
eukaryotes acquired the ability to build different proteins by reshuffling
smaller modules.

Hartman has his own theory on how complex cells came about. After studying
proteins in eukaryotes that aren’t present in simple cells, he has concluded
that there must be a third, long-extinct type of cell—he dubs it a
“chronocyte”—that fused with archaea and bacteria to form complex
cells.