Myosin gene mutation correlates with anatomical changes in the human lineage

Stedman et al,
University of Pennsylvania

Literature talk by Nick Fankhauser

The MYH16 gene diverged long ago
from the other human
sarcromeric myosin genes.

Seven monkey species
have an intact version
of MYH16, but humans
from all continents
have a frameshift
mutation that leads
to a truncated protein.

These are
monkey (M. fascicularis)
myofibres from
jaw muscle,
stained red
with immuno-
fluorescence.

Human jaw muscle
myofibres
are 8x smaller!

• Due to the decreased size of their jaw muscles, humans need a much smaller region of their skull for attachment of these muscles.


• This removed an evolutionary constraint that prevented the human brain from getting bigger.

• Gene inactivation
  happened 2.4Myr ago,
  estimated by alignment
  of orthologues.
• In the left branch of
  the tree, mutations are
  mostly silent.
• Sequence loses
  evolutionary
  conservation in
  human branch.

Human fossil history

• From more than 2.4 Myr ago, only chimp-like Australopithecus fossils are known. They had small brains, strong teeth and jaws.


• No Homo habilis fossil older than 2.4 Myr was found. His brain is one third bigger than that of Australopithecus and he used tools.
• Less than one Myr later H. erectus appeared with a brain twice that size and the ability to control fire.