In our cell biology class on placenta and human reproduction today, the
professor talked a bit about twinning. According to him, the rate of
monozygotic (identical) twinning is fairly constant across populations, but the
rate of dizygotic (fraternal) twinning varies considerably, depending
on maternal characteristics. In order to have dizygotic twins, a woman must at
least ovulate two eggs, at least two of which must be fertilized and be
implanted in the woman's womb. The following are true of the rate of dizygotic
twinning:
1. More likely among overweight women.
2. More likely among
women in their 30s.
3. More likely among women of African ancestry, and least
likely among Asians. Whites are in between. According to wikipedia, the
twinning rate of the Yoruba people in Africa is 40 per thousand. That of
the Japanese is 6 per thousand.
#1 makes perfect evolutionary sense. In
the evolutionary environment, being overweight was a signal that nutrition is
abundant. It would have been evolutionarily advantageous to have twins when
food was aplenty. Similarly, overweight children tend to enter puberty at an
earlier age. In contrast, severe dieting often delays puberty in teenagers and
often causes women to skip an ovultion. The human body is designed to reproduce
more in times of plenty, and reproduce less in times of famine.
#2
is paradoxical. I can't come up with an evolutionary justification. Could it
be that women in their 30s tend to have more dizygotic twins primarily because
they are more likely to use In Vitro Fertilization to become pregnant than
women in their 20s? In IVF, it is a routine practice to introduce two or more
fertilized eggs into the mother, because the natural failure rate for
implantation onto the womb is very high (majority of fertilized embyos do not
successfully implant).
#3 is partly driven by the tendency of Asians
to be thinner. But there was almost certainly selection pressure against
twinning in Asians and (to a lesser extent) in White people, given the
humongous variation in the rate of twinning and the widespread distribution
of the gap. All human populations outside of Africa descend from a small share
of prehistoric humans that left Africa - our species' birthplace. In many ways, Africans are our species genetic "default." Since East
Asia and Europe were relatively inhospitable, cold, dry places during the Ice
Age when humans first populated them, it may have been particularly disadvantageous to attempt to care for two babies at the same time. Perhaps for the same reason, Whites and
Asians hit pubterty about 2 years after people of African ancestry do.
