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Each equation, a line, in a stanza, within a poem that is the universe - (physicists-need-love-too)

Nictitating Membrane: a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining vision 

What is the evolutionary benefit or purpose of having periods? Why can’t women just get pregnant without the menstrual cycle?

Suzanne Sadedin, Ph.D. in evolutionary biology from Monash University

I’m so glad you asked. Seriously. The answer to this question is one of the most illuminating and disturbing stories in human evolutionary biology, and almost nobody knows about it. And so, O my friends, gather close, and hear the extraordinary tale of:

HOW THE WOMAN GOT HER PERIOD

Contrary to popular belief, most mammals do not menstruate. In fact, it’s a feature exclusive to the higher primates and certain bats*. What’s more, modern women menstruate vastly more than any other animal. And it’s bloody stupid (sorry). A shameful waste of nutrients, disabling, and a dead giveaway to any nearby predators. To understand why we do it, you must first understand that you have been lied to, throughout your life, about the most intimate relationship you will ever experience: the mother-fetus bond.

Isn’t pregnancy beautiful? Look at any book about it. There’s the future mother, one hand resting gently on her belly. Her eyes misty with love and wonder. You sense she will do anything to nurture and protect this baby. And when you flip open the book, you read about more about this glorious symbiosis, the absolute altruism of female physiology designing a perfect environment for the growth of her child.

If you’ve actually been pregnant, you might know that the real story has some wrinkles. Those moments of sheer unadulterated altruism exist, but they’re interspersed with weeks or months of overwhelming nausea, exhaustion, crippling backache, incontinence, blood pressure issues and anxiety that you’ll be among the 15% of women who experience life-threatening complications.

From the perspective of most mammals, this is just crazy. Most mammals sail through pregnancy quite cheerfully, dodging predators and catching prey, even if they’re delivering litters of 12. So what makes us so special? The answer lies in our bizarre placenta. In most mammals, the placenta, which is part of the fetus, just interfaces with the surface of the mother’s blood vessels, allowing nutrients to cross to the little darling. Marsupials don’t even let their fetuses get to the blood: they merely secrete a sort of milk through the uterine wall. Only a few mammalian groups, including primates and mice, have evolved what is known as a “hemochorial” placenta, and ours is possibly the nastiest of all.

Inside the uterus we have a thick layer of endometrial tissue, which contains only tiny blood vessels. The endometrium seals off our main blood supply from the newly implanted embryo. The growing placenta literally burrows through this layer, rips into arterial walls and re-wires them to channel blood straight to the hungry embryo. It delves deep into the surrounding tissues, razes them and pumps the arteries full of hormones so they expand into the space created. It paralyzes these arteries so the mother cannot even constrict them.

What this means is that the growing fetus now has direct, unrestricted access to its mother’s blood supply. It can manufacture hormones and use them to manipulate her. It can, for instance, increase her blood sugar, dilate her arteries, and inflate her blood pressure to provide itself with more nutrients. And it does. Some fetal cells find their way through the placenta and into the mother’s bloodstream. They will grow in her blood and organs, and even in her brain, for the rest of her life, making her a genetic chimera**.

This might seem rather disrespectful. In fact, it’s sibling rivalry at its evolutionary best. You see, mother and fetus have quite distinct evolutionary interests. The mother ‘wants’ to dedicate approximately equal resources to all her surviving children, including possible future children, and none to those who will die. The fetus ‘wants’ to survive, and take as much as it can get. (The quotes are to indicate that this isn’t about what they consciously want, but about what evolution tends to optimize.)

There’s also a third player here – the father, whose interests align still less with the mother’s because her other offspring may not be his. Through a process called genomic imprinting, certain fetal genes inherited from the father can activate in the placenta. These genes ruthlessly promote the welfare of the offspring at the mother’s expense.

How did we come to acquire this ravenous hemochorial placenta which gives our fetuses and their fathers such unusual power? Whilst we can see some trend toward increasingly invasive placentae within primates, the full answer is lost in the mists of time. Uteri do not fossilize well.

The consequences, however, are clear. Normal mammalian pregnancy is a well-ordered affair because the mother is a despot. Her offspring live or die at her will; she controls their nutrient supply, and she can expel or reabsorb them any time. Human pregnancy, on the other hand, is run by committee – and not just any committee, but one whose members often have very different, competing interests and share only partial information. It’s a tug-of-war that not infrequently deteriorates to a tussle and, occasionally, to outright warfare. Many potentially lethal disorders, such as ectopic pregnancy, gestational diabetes, and pre-eclampsia can be traced to mis-steps in this intimate game.

What does all this have to do with menstruation? We’re getting there.

From a female perspective, pregnancy is always a huge investment. Even more so if her species has a hemochorial placenta. Once that placenta is in place, she not only loses full control of her own hormones, she also risks hemorrhage when it comes out. So it makes sense that females want to screen embryos very, very carefully. Going through pregnancy with a weak, inviable or even sub-par fetus isn’t worth it.

That’s where the endometrium comes in. You’ve probably read about how the endometrium is this snuggly, welcoming environment just waiting to enfold the delicate young embryo in its nurturing embrace. In fact, it’s quite the reverse. Researchers, bless their curious little hearts, have tried to implant embryos all over the bodies of mice. The single most difficult place for them to grow was – the endometrium.

Far from offering a nurturing embrace, the endometrium is a lethal testing-ground which only the toughest embryos survive. The longer the female can delay that placenta reaching her bloodstream, the longer she has to decide if she wants to dispose of this embryo without significant cost. The embryo, in contrast, wants to implant its placenta as quickly as possible, both to obtain access to its mother’s rich blood, and to increase her stake in its survival. For this reason, the endometrium got thicker and tougher – and the fetal placenta got correspondingly more aggressive.

But this development posed a further problem: what to do when the embryo died or was stuck half-alive in the uterus? The blood supply to the endometrial surface must be restricted, or the embryo would simply attach the placenta there. But restricting the blood supply makes the tissue weakly responsive to hormonal signals from the mother – and potentially more responsive to signals from nearby embryos, who naturally would like to persuade the endometrium to be more friendly. In addition, this makes it vulnerable to infection, especially when it already contains dead and dying tissues.

The solution, for higher primates, was to slough off the whole superficial endometrium – dying embryos and all – after every ovulation that didn’t result in a healthy pregnancy. It’s not exactly brilliant, but it works, and most importantly, it’s easily achieved by making some alterations to a chemical pathway normally used by the fetus during pregnancy. In other words, it’s just the kind of effect natural selection is renowned for: odd, hackish solutions that work to solve proximate problems. It’s not quite as bad as it seems, because in nature, women would experience periods quite rarely – probably no more than a few tens of times in their lives between lactational amenorrhea and pregnancies***.

We don’t really know how our hyper-aggressive placenta is linked to the other traits that combine to make humanity unique. But these traits did emerge together somehow, and that means in some sense the ancients were perhaps right. When we metaphorically ‘ate the fruit of knowledge’ – when we began our journey toward science and technology that would separate us from innocent animals and also lead to our peculiar sense of sexual morality – perhaps that was the same time the unique suffering of menstruation, pregnancy and childbirth was inflicted on women. All thanks to the evolution of the hemochorial placenta.

https://www.quora.com/what-is-the-evolutionary-benefit-or-purpose-of-having-periods

So how bad is Guns, germs, and Steel? Is it still worth reading or do people consider it bunk/ too pop history.

It is a garbage book. Diamond tries to answer difficult questions, but in doing so simplifies issues and topics to the point of being wrong. This is compounded by his use of out of date information and poor understanding of topics within anthropology. Fans of his book like to say it is just sour grapes among academics, but his fans are the same sort of people who think colonialism was justified and that we no longer need to do anything for contemporary Native peoples.Instead, I recommend reading 1491 by Charles Mann. He presents a lot of information, much of it very new and groundbreaking at the time of its publication in 2006. It is a favorite among academics to assign to their intro classes because Mann does not try and interpret research to fit a narrative like Diamond does. His sequel, 1493, is equally great with his discussion on the effects of the the Columbian Exchange. My favorite topics are the spread of the potato to Europe, the lack of European use of Native agricultural practices for cultivating the potato, and the resulting potato famines as a result of using New World guano as fertilizer which introduced a fungus that killed the potato. The effects of the famines were compounded by the use of European agricultural practices for the potato which greatly increased the spread of the fungus. If they had used Andean potato cultivation practices the potato famines may not have been so severe. My other favorite topic is how all the silver wealth extracted by Spain fro the New World initially made them rich, but the continued introduction of silver quickly caused a financial crisis with rampant inflation. Spain’s response was to use more silver. All this silver made its way to China which destabilized China’s economy and left it open for later colonization efforts.

There are SO many types of coders. Do any of these remind you of someone you know? 🤔  Tag em! 

Coronal section 💕

Feeding the gods: Hundreds of skulls reveal massive scale of human sacrifice in Aztec capital.

Fun Fact: The obsidian blade used during the rituals are sharper than today’s surgical steel.

The image below shows a reconstruction of Tenochtitlan, the capital of the Aztecs. Templo Mayor is the pyramid seen center-back, with two temples on top. One temple was dedicated to the war god, Huitzilopochtli, and the other to the rain god, Tlaloc.

Image Source: Rosemania. Reconstruction of Tenochtitlan, the capital of the Aztecs - the centre of modern Mexico City. (National Museum of Anthropology of Mexico City). Wikimedia Commons.

The enormous rack of skulls called, Tzompantli, was built in front of the Templo Mayor pyramid. The Mexica people performed human sacrifices to feed the gods. The Aztecs made up the majority of the Mexica people. To them, the skulls would guarantee the sustained continuation of humanity.

There are numerous depictions of tzompantli in Aztec codices. Here is one taken from the 16th Century Aztec manuscript, Codex Duran.  Image source: Public Domain.

The Spanish conquistadors marched into Tenochtitlan in 1519. The Spanish saw the skulls and the practice of human sacrifice as barbaric. In 1521 the Templo Mayor was torn down, and the tzompantli paved over. The ruins are underneath of what is known today as Mexico City. Archaeologists are currently studying the skulls to learn about rituals and the postmortem handling of those that were sacrificed.

pls disragard these questions if you arent feeling like explaining anthropology that is probably not within your specific field, but i have questions and somehow youre the most accessible source for answers about ancient-ass humans. so: why are humans so much hardier than other animals? like if you break a horse's bone that horse is kaput, but people bounce back from shit like missing limbs. how are we so cool? also, how prevalent (and when) was pursuit predation? also, thanks! have a nice day

OH no worries, this is something I teach every year and it’s REALLY COOL. There’s footnotes and works cited below the jump if you want them, and I can point you at some other work if this is something you’re interested in reading about. Humans are ridiculously resilient. The reason for this has a lot to do with the tradeoffs we made for endurance rather than speed. Human walking is really energy efficient (it’s really just controlled falling) compared to a horse’s galloping, and we have pretty well-muscled legs. Our plantigrade feet mean that there’s not as much energy when we spring off compared to an animal that runs on their toes, but at the same time, we spend a lot less energy moving around. 

Our muscles and leg anatomy have a lot to do with it, too. Let’s look at a horse’s leg compared to a human’s.

Horses in particular have a hard time with broken legs because they have a LOT of mass resting on on those legs. Horses’ legs are basically built to go fast- their leg bones are actually quite light, and below the ankle there’s… well, basically nothing. Just tendon and skin- there’s no big muscles to stabilize or cushion the bone. This means that there’s less weight to drag around so the horse can escape a predator more quickly, but it comes with a major tradeoff- if a horse breaks their lower leg, it tends to shatter. In the wild, this is going to make the horse extremely easy to pick off. But like I said up there, humans- unlike horses- don’t run on our toes. Our ankle bones are chunky and strong, and our lower legs are cushioned with muscle and fat. But our healing ability goes beyond just basic anatomy! Our group dynamics also play into this, too. If a horse breaks a leg, what can the other horses do about it? The injured horse still has its biological needs to fulfill; it has to eat, drink, and evade predators. It has to keep moving- it can’t lie down for a few weeks and let the leg heal. But that’s not true of humans and our closest relatives! I’ll use Neanderthals for this example because I like them a lot, but the same goes for early modern humans, too. 

Let’s say that some Neanderthals are out on the hunt, and Thog gets knocked against a tree trunk by a mammoth and she breaks her leg. Because Thog’s a member of a social species, it’s not the end of the world for her or her group. The rest of her crew can keep hunting and Thog can limp back home, where her grandfather looks after her and her younger sister brings her water. She’s able to rest and keep weight off the broken leg, which means that so long as she keeps whatever wounds there might be relatively clean, sepsis is less likely. Group living means that you don’t have to be self-sufficient; no hominin is an island. Part of why we’re so successful is that our ability to care for each other ensures better group survival. If your reproductive-age individuals are also providers, group care means that you’re less likely to lose them. 

We know from looking at Neanderthal skeletons that they were injured frequently and were able to shake it off and survive; even elderly individuals with severe arthritis are often found in group contexts, suggesting they weren’t left behind. And we are talking serious injuries here- not just broken legs, but head and neck trauma, too. There’s a famous paper* that says that most Neanderthal injuries came from close contact hunting (due to them being mostly head and neck injuries rather than lower body injuries), but more up-to-date research notes that actually, Neanderthals could- and did- get hurt pretty much everywhere**. 

As to when pursuit predation came into effect, the best guess we have is “probably sometime around two million years ago, practiced by Homo erectus/ergaster.” One way we can tell this is by diet. Mandibles are very quick to adapt to dietary pressures, so by comparing mandibles to things with known diets, we’re able to tell what’s going on. Add that to dental wear and morphology studies and chemical analysis of subfossils’*** teeth and we can get a pretty good picture of who’s eating what. What we see with the H. erectus/ergaster complex of species is that they’re eating a wide variety of very tough foods; their jaws and molars suggest that they were eating root vegetables, tough meat, tubers, bone marrow, honey- really, anything they could find. We also know that they were eating a fairly high calorie diet compared to their predecessors; this was necessary for brain development- and we know that these calories came from meat. As average brain mass increases, so does evidence of meat-eating. Brain development is expensive- you have to put a lot of nutrients into it- nutrients that are really hard to get from plants alone. One way to feed the family is by hunting- though realistically this didn’t happen all that regularly. Rather, it’s more likely that hunting supplemented gathering, as it does with many forager groups today; hunting takes a lot of energy and can be dangerous.Archaeology also points to the “around two million years ago” date based on stone tool deposits and fossil prey species. One good example of this is Kanjera in Kenya; it’s a site by Lake Victoria that has good evidence for persistence hunting. There’s quite a lot of gazelle and antelope skeletons that aside from stone tool marks, don’t have a lot of damage. It looks like these were brought to the site for butchering- and they would have had to be hunted because hyenas, lions, and other predators and scavengers will actually eat those bones. Gazelles are a lot faster than humans, and the hominins of the time didn’t really do projectiles; rather, it’s more likely they ran the gazelles down until they were exhausted, then dragged them back to this lakeside camp site to process and eat. It’s likely that this strategy helped fuel the migration of Homo into Asia; once you’re able to hunt big game, your movement is less restricted by the availability of small animals, scavenged meat, and seasonal plants; you can follow herd animals and just chase one whenever you need to eat. However, the exact role that hunting and scavenging played in the development of the Homo genus is something that archaeologists and physical anthropologists do not agree on- when you’re trying to figure out what a species eats and how they got their food, you gotta realize that this can vary heavily by what’s available, what predators are in the area, your own group’s composition, and myriad other factors that can affect food acquisition. 

One thing we do know for sure: Persistence hunting works. Our species is super good at it, even today. If you’d like to see some persistence hunting in action, it’s actually still used by San groups in Africa.

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