恐竜の絶滅はメス不足のため?


恐竜の絶滅は、地球環境の変化が性差によるメスの不足から来たという説を、批判的に紹介する記事です。

Did Dinosaurs Die Out Because Males Couldn’t Find a Date?

What caused the end-Cretaceous mass extinction is one of the greatest mysteries of all time. Paleontologists have racked up a long list of victims—including the non-avian dinosaurs—and geologists have confirmed that a massive asteroid that struck the earth near the modern-day Yucatan peninsula was probably the extinction trigger, but just how that impact translated into a global extinction crisis is still being figured out. Of course, dinosaurs were the most charismatic creatures to perish during the event, and for years Sherman Silber has been forwarding his own peculiar proposal.

今からおよそ6500万年前の恐竜の絶滅。その引き金になった出来事のひとつに、隕石の衝突がありました。もっとも、実際のところ、それがどのように、またどのくらい恐竜絶滅に影響を与えたかについては、議論が尽くされていません。

隕石の影響をとなえる仮説のひとつに、恐竜の卵は、ワニのように、温度によって雄か雌かが決まるメカニズムになっており、隕石衝突後の温度変化でメスが生まれにくくなり、オスの繁殖相手がいなくなったことが原因というものがあります。

もっとも、この説に従うなら、たとえばカメも同じように温度によって性別が決まるのに、カメは中生代から現在まで生き延びたのはどうしてかなど、矛盾し説明できない点が残ります。

そんなわけで、環境変化がオスとメスの個体数の差を生んで、子孫が残せなかったというのはちょっと無理筋かな、という記事です。

仮説関連の論文は下記。

Human male infertility, the Y chromosome, and dinosaur extinction

Study of the molecular genetics of human male infertility and the Y chromosome has helped to elucidate the evolution of our X and Y chromosomes. Particularly, the study of the Y chromosome in male infertility has also helped to clarify, in a surprising and unexpected way, a likely mechanism for dinosaur extinction, the biggest question all of us have entertained from our earliest childhood days.

There have been many claims in the popular press of “discoveries” on how the dinosaurs went extinct. These claims all relate to climate change events that occurred 65 million years ago that no one disputes occurred. But none have explored the biology of how so many animals escaped extinction while the dinosaurs and at least half of all other species did not. For example, why did large dinosaurs, as well as small dinosaurs the same size as chickens go extinct, but birds survived? Possibly the evolution of sex chromosomes holds the answer to this question.

Our studies of the Y chromosome and male infertility suggest that the default mechanism for determining the sex of offspring is the temperature of egg incubation, and that genetic sex determination (based on sex chromosomes like X and Y) has evolved many times over and over again in different ways, in different genera, as a more foolproof method than temperature variation of assuring a balanced sex ratio in offspring. The absence of such a genetic sex determining mechanism in dinosaurs may have led to a skewed sex ratio when global temperature dramatically changed 65,000,000 years ago, resulting in a preponderance of males, and consequentially a rapid decline in population.

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