Boston
THE secret to what makes us human may lie not in our brains, but in sperm.
Tiny changes in these little cells may have made all the difference to our
evolution.
DNA is packed into sperm heads by proteins called protamines, which help to
squash the chromosomes into a tiny space. These proteins were never considered
very important in human development because people thought their influence
stopped shortly after fertilisation. But Caro-Beth Stewart at the University of
Albany, State University of New York says these proteins evolved with lightening
speed just as humans split from the common ancestor of animals like gorillas and
chimps, indicating that the proteins have a much bigger role in human
development than previously thought.
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Researchers are constantly on the hunt for ways of explaining why humans are
so different, and so much brighter, than our close relatives
(快猫短视频, 23 February, p 15).
Chimpanzees and humans parted ways only 5
million years ago, leaving us with 98.5 per cent of the same DNA. Working out
how the remaining 1.5 per cent of our genetic make-up can explain all the
differences between species is proving tough.
Stewart鈥檚 team looked for genes that have changed dramatically since the
evolutionary split. They found that one group of genes has evolved remarkably
quickly: protamine 1, protamine 2, protamine 3 and
histone H1t. These are all genes that code for proteins involved with
binding sperm DNA. Protamine 1, for example, now codes for a protein
sharing only 84 per cent of the same amino acids with chimps. Changing 16 per
cent of the amino acids in 5 million years is 鈥渟creamingly fast evolution鈥�, says
Stewart.
In most other primates, protamines have not changed at all over this period.
That鈥檚 not too surprising, says Stewart. Sperm performs a vital task and
tinkering with its DNA is more likely to cause harm than good. She thinks the
new proteins in human sperm were probably specifically selected for because they
gave some kind of advantage.
These crucial changes may have altered how protamines bind DNA. Because these
proteins are present at the very moment of fertilisation when an embryo鈥檚
genetic material is first being programmed, Stewart guesses that even small
changes in their function might have huge consequences for the fate of the
embryo. Different protamines could set off a different pattern of gene
activation during development, she says, even in genes that are identical
between species.
Stewart points out that other recent studies suggest that protamine genes are
also switched on in fetal brains and adult human brainstems. So the proteins
might continue to affect us even after the embryo stage.
To understand how these changes have shaped humans, Stewart will study
exactly how protamine proteins bind and shape DNA in different primate species.