Heteropian twins or "by-products" of evolution

eggs may be a by-product of the "insurance strategy" that women evolve with age.
is an evolutionary paradox of the birth of an alien twin, a sibling produced simultaneously by multiple eggs. Mothers who give birth to twins usually have higher levels of health, but although twins are genetic, they are born with very low frequency in humans.
ovaries usually release one egg during an ovulation period, but occasionally two, which can lead to the birth of identical twins. Women's chances of having identical twins increase with age, peak in their 30s, and then decline.
to explain this phenomenon, Wade Hazel of the University of Diborg in Indiana and Joseph Tomkins of the University of Western Australia and colleagues speculated that as women get older, they are more likely to lay out two eggs in a cycle. This strategy prevents an increased risk of loss of prenatal embryos, which are more common in older women.
the researchers modeled the frequency of the appearance of brother twins and compared their predictions with the actual "twin ratio" of nine populations. If the model includes double ovulation rates that increase with age and fetal survival rates that decrease with age, then the twin rates of women of all ages can be best explained.
that while rowing two eggs at a time is an insurance policy for fertility, sometimes both embryos survive - and twins are born. (Source: Feng Weiwei, China Science Journal)
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