- A gene that separates humans from the apes and all other animals seems
to have disappeared from humans up to three million years ago, just before
they first stood upright, researchers said on Monday.
Most animals have the gene but people do not -- and it may be somehow
involved in the expansion of the brain, the international team of researchers
said. The gene controls production of a sialic acid -- a kind of sugar
-- called Neu5Gc, the researchers write in an advance online issue of
the Proceedings of the National Academy of Sciences.
"This mutation occurred after our last common ancestor with bonobos
(pygmy chimpanzees) and chimpanzees, and before the origin of present-day
humans," they wrote. Neanderthal skeletons, the oldest early humans
from who DNA has been obtained, also lack the sugar.
"It happens to be first known genetic difference between humans and
chimpanzees where there is a major outcome," Ajit Varki of the University
of California San Diego, who led the research, said in a telephone interview.
"We are exploring the consequences of this." Varki said the
role of the gene is not fully understood.
"The gene itself is involved in changing the surfaces of all cells
in the body," he said. "The surface of all cells in the body
is covered with sugars. This one is missing only in humans."
It may help influence how viruses and bacteria infect cells, and with
how cancer cells interact, Varki said. "There are some clues that
it might have something to do with brain plasticity," he added.
Humans and chimps share more than 98 percent of their DNA, so a few genes
must make a big difference. Chimps and humans split from a common ancestor
6 million to 7 million years ago.
The collaboration of some of the top experts in various fields, ranging
from anthropology to the genetic differences between people and apes,
determined that this gene disappeared from humans between 2.5 million
and 3 million years ago.
"It happened after the time that our ancestors stood upright, when
their hands and so on were like ours, but their brains are still same
size as that of chimpanzees," Varki said.
"That just tells you the timing is appropriate for the possibility
that this may have something to do with brain expansion."
The team included anthropologist Meave Leakey of the Leakey Foundation
in Nairobi, Kenya, an expert in early humans, and Svante Paabo of the
Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany,
who helped study the first Neanderthal DNA.
Earlier this month Paabo's team reported that the had found mutations
in a gene called FOXP2 that seems to be involved in the face and jaw movements
necessary for speech. A relatively small change makes the human version
of the gene different from the version found in apes, the researchers
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