THE NUCLEUS Issue 2 Spring 2024

Here’s a fun fact: Non-mammal red blood cells have nuclei. Some scientists believe this is because early mammals and their ancestors (synapsids) needed to evolve for greater speed and mobility (our limbs being under us rather than to the side, supports this theory) and thus higher oxygen capacity was obviously necessary.

SO WHY DO WOMEN BLEED?

Why do women bleed? One of the most culturally fascinating types of blood is menstrual blood. Humans are one of only four mammal groups that actually have menstrual blood (apes and old-world monkeys, some bats, elephant shrews, and one rodent species). Most of the other mammals still have menstrual cycles; they just reabsorb the old womb-lining rather than bleed it out. To this day, no one understands why women bleed. However, there are some theories. A feature this seemingly random set of mammals all share is that, for a large part of their evolution, they were either arboreal or flew. Therefore, the main negative evolutionary effect of menstrual bleeding (being easily trackable by predators) did not exist, and as it is a relatively insignificant amount of blood in comparison to the total amount in the body, there was not that much else stopping it from happening. Thus, if a mutation for menstrual bleeding arose, there was no evolutionary disadvantage, and thus they spread. This also aligns with the fact that not all historically flying mammals bleed every month; thus, if menstrual bleeding arose in any other non-arboreal mammal, it would be selected against, but in arboreal mammals, it has a chance of surviving the test of time.

So, next time you get a cut, take a step back and think of everything that went into making your blood.

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