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Learn about the history of dudes, and a new theory about deep voices in this new episode of SciShow News. You know, science... bro... stuff.

Hosted by: Hank Green
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Sexual Dimorphism

[SciShow Intro plays]

Hank: This week’s SciShow News is all about the evolution of the modern male human. Science bro stuff. So let’s start with a new model of how the Y chromosome evolved across the globe.

In a paper published this week in Nature Genetics, a team of 42 international researchers performed a huge study of variations in the human Y chromosome. Their goal? Modeling the history of men over thousands of years.

By studying the Y chromosome, which is passed on from fathers only to their biologically male children, geneticists can predict how male populations changed and spread out over time. That helps us understand how different humans evolved -- including population explosions, migrations, and common ancestors -- one DNA sequence at a time. Scientists can decode the history of humans to show evolutionary relationships using different types of DNA. These researchers chose to focus on Y chromosomes. And in total, they analyzed over 1200 DNA sequences from 26 populations worldwide.

Specifically, they were looking for different types of variations in the DNA -- things like tiny changes in the sequence or sections that were accidentally repeated or deleted. Once they recorded all of these variations, they sorted people with more similar Y chromosomes -- and therefore more likelihood of sharing a common ancestor -- into categories called haplogroups. And based on these haplogroups, they used mathematical models to come up with an evolutionary, or phylogenetic, tree.

The tree essentially shows how different groups of males evolved from one another over time, passing down slight variations of the Y chromosome. Their data supported previous studies on Y chromosome haplogroups, and also revealed more information about possible branching patterns. Like, when they added an estimated mutation rate into their calculations, they were able to work backward, tracing the modern Y chromosome back to a single dude about 190,000 years ago.

They also found evidence of population explosions at specific times and places. These bursts of new populations could be linked to things like migration, when people spread out and colonized new continents -- like around 50,000 years ago, when humans spread across Asia and Europe. They could also be linked to new technology -- things like organized war or better transportation that could affect which guys got to have families and where.

Now, these connections are still hypothetical. They’re based on models of DNA mutation rates, our records of historical events, and some educated guesswork. But this research does give us a pretty clear picture of how humans might have evolved. And with more types of DNA analysis from a broader population over time -- and especially from older human remains -- we can continue refining that picture.

But our genetics do more than just tell us how we evolved over time. They also determine some of our characteristics. And sometimes, those characteristics are specific to either males or females of a species. You know how male peacocks have brightly colored feathers to show off and attract the ladies, while females are brown and smaller? That difference is one example of sexual dimorphism. And in a paper published this week in the Proceedings of the Royal Society B: Biological Sciences, a group of researchers studied sexual dimorphism in humans and other primates -- specifically, in their voices.

Turns out, deeper voices are about more than just macho-ness: they could also be a sign of a well-functioning immune system. For lots of primates, males have larger vocal cords, and therefore much deeper voices, than females. It’s possible that these deep booming calls are to exaggerate the appearance of body size, or intimidate other males while they’re trying to woo the ladies. And even studies in humans seem to show that deeper voices -- for some reason -- make men seem more attractive and dominant to heterosexual women.

But this doesn’t really make intuitive sense. You don’t necessarily have to be strong, or big, or formidable to have a deep voice -- which all would’ve been attractive qualities in a survival-focused world. So why do heterosexual human females seem to have this preference for deeper-voiced males? The researchers conducted three separate studies to try to find a biological reason for it. The first two studies confirmed what we already knew from previous research: First, that primates seem to have more dimorphism in voice when there’s more competition for mates. And second, that lower voices in male humans seemed to make them more attractive. But their third study found that vocal pitch can be related to hormones.

In males, a deeper voice tends to come with higher testosterone levels. And the researchers found that deeper-voiced males had both higher levels of testosterone, and lower levels of a hormone called cortisol, which is involved in metabolism and stress response. Generally, when there’s more cortisol in your bloodstream, like in response to an infection, it inhibits the effects of testosterone. So higher overall testosterone levels may mean lower amounts of cortisol are being produced and getting in the way of that testosterone -- which implies less stress and less illness.

Since hormones can affect health, selecting for a better hormone profile and well-functioning immune system would be an evolutionary benefit -- which would explain why deeper-voiced men might be seen as more attractive. It’s just another one of those things that signals to the world around you “hey, I’m healthy.”

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