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           PHOTO:
The difference between chimps and humans is all in the mind. It is
differences in our brain's gene activity that really sets us apart from
chimps, delegates at the Human Genome Meeting in Edinburgh, Scotland,
heard this week.

"I'm interested in what makes me human," explains Svante Pääbo of the Max
Planck Institute for Evolutionary Anthropology in Leipzig, Germany. After
sequencing 3 million letters of the chimp genome and comparing them with
the human draft, his group reasoned that DNA sequence can't be it: only
1.3f letters are different.

So using tiny 'gene chips' with 20,000 human genes dotted on them, they
measured the levels of gene activity, or 'transcription', in our brain,
liver and blood. They compared these transcription snapshots — the
'transcriptome' — with similar snapshots of our close relative the chimp
and an evolutionarily more distant relative, the rhesus macaque monkey.
"Liver and blood reflect how the species are related," Pääbo found. In
these tissues, as expected, the human gene activity pattern was pretty
similar to that of the chimp, and different from the macaque.

The brain showed a different picture: chimp and human transcription
patterns are poles apart. "The [human] brain has accelerated usage of
genes," explains Pääbo.

The genomes of all mammals are so similar that "it's hard to understand
how they can produce such different animals", says Sue Povey, who works on
human gene mapping at University College London in England. If their genes
are alike, it's probably changes in when, where and how active they are
that drives the differences between species, she agrees.

Comparing our genome with those of other animals is generally agreed to be
key to making sense of the mass of human sequence information. "The best
way to see what's important is to see what's different between human and
mouse or human and fish," Eric Lander, one of those sequencing the human
genome, has said. Lander works at the Whitehead Institute/MIT Center for
Genome Research in Cambridge, Massachusetts.

But it is comparisons with our close relatives that will really highlight
what makes us special, thinks Pääbo: "they point to features unique to
humans." And his findings suggest that "the head may be the most fruitful
place to look".

To understand these genetic differences, however, we also need to know
what's going on in a chimp's mind; after all, we know that chimps have
basic language and learning skills. "We like to think we have higher
levels of thought than chimps," says Povey. "But we know what we think and
we don't know what they think."

Human Genome Organisation
Human Genome Meeting 2001
The Human Genome

A human skull (top) houses more gene activity than does a chimp skull (bottom)