Re: What makes us human?
- From: "Ron O" <rokimoto@xxxxxxx>
- Date: 23 Aug 2006 04:24:57 -0700
Jenny Brien wrote:
From a Daily Telegraph article of the same name
http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2006/08/01/echuman01.xml&page=4
<quote>
Chimps and humans each have about three billion nucleotides in their
genomes - 99 per cent of those may be identical, but that still leaves
about 30 million differences. Most of those are unimportant, the
background noise of genomic evolution. But some matter: which?
Therein lies the importance of microcephaly. The discovery of genes
that control the growth of the brain immediately suggested that these
genes might also have changed in the last six million years since we
last shared an ancestor with chimps. And so it proved: of the four
microcephaly genes that have been found, three bear the hallmarks of
rapid evolution. To be sure, chimps have versions of these genes, but
the human version is different. So different, in fact, that their
evolution must have been driven by natural selection.
</quote>
O.K. As a lay person I can understand that. Natural selection
preserves favourable mutations. "Favourable" for humans includes bigger
brains, so the genes which affect that are going to change more than
other genes, simply because favourable mutations are passed down to
future generations.
What I don't yet understand is this: I watched the program of the same
name on Channel 4, on which the author of the article, Dr Armand Leroi,
of Imperial College London, said that a particular mutation on one of
these genes first appeared after the invention of agriculture. Is that
simply a hypothesis, or is it indeed possible to date the first
appearance of a mutation so precisely and what is the thinking behind
it?
A simplistic explanation is that they can see that the mutation is
imbedded in mutations that happened much longer ago. Say the chimp and
human gene sequence changed by 5 important mutations A', B', C', D',
and E'. There are probably many more differences in the gene between
chimps and humans because these brain genes tend to be big and our
sequence varies by around 1% in the coding regions. Everyone has
haplotype A', C' and D' with those three mutations. A haplotype is
just the sequence of one of a pair of chromosomes. They come in pairs
so everyone has two haplotypes for a particular gene or chromosomal
region. In some populations they find that some humans have an
additional mutation that must have occurred after A', C', and D'. It
might be A', B', C' and D', Still others might have all 5 and the
fifth mutations E' can be inferred to have occurred after all the
others.
I'll also say that these mutations might not be "good" things if they
have happened after the rise of agriculture, and since 40,000 years
ago. Human brains used to be bigger in the early hunter gatherers.
Neandertals and Cro Magnon had larger cranial capacities than we do on
average. Agriculture and technology may have allowed us to survive
with less brains. Brains are expensive to maintain, and if you don't
need a large one there is nothing to keep it large. Just like eyes
degenerate in cave fish when they no longer need their eyes because
they can't see anything. We might get a hint about what direction
these mutations are taking humans by looking at the frequency of these
mutations among the creationists on the Kansas, Ohio and Dover school
boards that wanted to do what they tried to do. It would depend on
whether they were honestly incompetent and/or actively dishonest, so it
wouldn't be definitive.
Ron Okimoto
.
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- What makes us human?
- From: Jenny Brien
- What makes us human?
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