Ceilidh

Michael Zey
futurist3000@aol.com

ISSN:                     00999660

Subject Terms:            Deoxyribonucleic acid
                         DNAGenomics
                         Monkeys & apes

Classification Codes:     9190: United States
                         5400: Research & development


Abstract:

Compared with other myosin genes, Dr. [Hansell Stedman] found, one such
gene (called MYH16) was missing two "base pairs," the molecular units that
make up DNA. At first he figured the missing base pairs was a rare mutation.
But the genome of every human he and his colleagues studied -- Africans
and South Americans, Europeans and Japanese -- has the mutation.

We also have something else. The larger the muscle, the more bone is required
to anchor it. As a result, gorillas and macaques with big jaw muscles also
have extra bone on top of their skulls. "The way the bone is laid down,
the cranium can't grow," says Dr. Stedman. But in humans, who don't need
extra skull bone, the skull keeps growing. And so does the brain inside
it. Only half in jest, the scientists call the muscle mutation RFT, "room
for thought."

If the jaw-muscle protein is any indication, though, scientists would do
well to look beyond the obvious suspects for the genetic changes that produced
humans from ape-like ancestors. "The myosin gene sat around for years"
without anyone realizing its significance, says Dr. Stedman. "Some of the
most fascinating genes accounting for the differences between humans and
apes might also be lying around."
Copyright (c) 2004, Dow Jones & Company Inc. Reproduced with permission
of copyright owner. Further reproduction or distribution is prohibited
without permission.

Full Text:

IF A CHIMPANZEE were one of those puzzles on the back of a children's cereal
box -- the ones where you're shown a shape composed of a dozen toothpicks
and asked to move only four of them to form, say, a star -- the instructions
would go something like this: Start with the thousands of genes in the
chimp genome. Change, delete or add only a few hundred so that you now
have the full genetic instruction manual for making a human.

Misapprehensions to the contrary, humans did not evolve from chimps. Both
species evolved from a common, apelike ancestor. Yet since the two lineages
diverged some five million years ago, the genetic changes have been few.
Chimps are our closest living relative. Their genomes are about 97% identical
to ours. How, then, can so few genetic differences account for our language-using,
cathedral-building, opera- writing and computer-inventing?

Yesterday, scientists announced that they have taken a big step toward
solving that mystery. Writing in the journal Nature, they report the discovery
of the first genetic difference between humans and chimps that produces
a clear functional difference. Making the finding more tantalizing, the
mutation occurred at roughly the same time that humanlike traits first
appeared in the fossil record. Yet the gene has nothing to do with intelligence
or other traits, at least not directly, and therein lies a lesson.

BACK WHEN the human genome project was going fast and furious, churning
out DNA sequences by the thousands every day, Hansell Stedman of the University
of Pennsylvania was looking for any genes that might affect muscle proteins
(a surgeon and molecular geneticist, his specialty is muscle diseases).
The most common muscle proteins are myosins; humans have about 10 genes
that produce one or another myosin.

Compared with other myosin genes, Dr. Stedman found, one such gene (called
MYH16) was missing two "base pairs," the molecular units that make up DNA.
At first he figured the missing base pairs was a rare mutation. But the
genome of every human he and his colleagues studied -- Africans and South
Americans, Europeans and Japanese -- has the mutation.

With more sleuthing, they found the last thing they expected in a little
project on muscle disease. Macaques and at least six other non- human primates
have a nonmutated version of MYH16, which gives them huge jaw muscles.
Humans' mutated gene keeps us from making this jaw- muscle protein, and
so we have smaller jaws.

But we also have something else. The larger the muscle, the more bone is
required to anchor it. As a result, gorillas and macaques with big jaw
muscles also have extra bone on top of their skulls. "The way the bone
is laid down, the cranium can't grow," says Dr. Stedman. But in humans,
who don't need extra skull bone, the skull keeps growing. And so does the
brain inside it. Only half in jest, the scientists call the muscle mutation
RFT, "room for thought."

"Something going on outside the brain turns out to have a very important
impact on what's going on inside it," says Dr. Stedman. "This was a surprise
to all of us."

USING STANDARD molecular genetics, the Penn team infers that the muscle-minimizing
mutation appeared 2.4 million years ago. That is right before fossils of
human ancestors start having larger, more human-like skulls. Before then,
human ancestors such as Paranthropus and Australopithecus had large jaws
and small skulls filled with small brains. Although the jaw-muscle mutation
was surely not the only, and maybe not even the most important, genetic
change that created the human species, it likely helped unleash the brain
growth that makes humans human.

The Penn discovery, writes developmental biologist Pete Currie of the Victor
Chang Cardiac Research Institute, Sydney, Australia, in a commentary in
Nature, "suggests that the genetic basis of human evolution can and will
be defined."

That will require a point-by-point comparison of the human and chimp genomes.
The sequencing of the human genome was finished last April. The chimp genome
is coming along; the first draft was released in December.

Now comes the needle-in-a-haystack part. Ajit Varki of the University of
California, San Diego, guestimates that there could be 40 million places
where the base pairs of the human genome differ from those of the chimp.
That's out of 3.1 billion base pairs.

So far, researchers have identified a handful of genetic differences between
humans and chimps. A gene involved in language development seems unique
to humans, researchers led by Svante Paabo of the Max Planck Institute
for Evolutionary Anthropology in Liepzig, Germany, reported in 2002. And
last year he found that humans have many fewer genes for olfactory receptors,
molecules that detect aromas, than great apes do.

If the jaw-muscle protein is any indication, though, scientists would do
well to look beyond the obvious suspects for the genetic changes that produced
humans from ape-like ancestors. "The myosin gene sat around for years"
without anyone realizing its significance, says Dr. Stedman. "Some of the
most fascinating genes accounting for the differences between humans and
apes might also be lying around."

---

You can e-mail me at sciencejournal@wsj.com.

(See related letter: "Letters to the Editor: Smells Can Revive Dormant
Memories" -- WSJ April 5, 2004)

(See related letters: "Letters to the Editor: Memory and the Senses --
WSJ April 14, 2004)

Reproduced with permission of the copyright owner.
Further reproduction or distribution is prohibited without permission