I love chimpanzees. I've always loved chimpanzees. My dad and my mom made sure I watched lots of nature documentaries growing up, so I remember those classic Nat Geo Jane Goodall docs. I fell in love with chimps from a very early age. I never thought I would be an evolutionary biologist or a professor. I never assumed as much.

When I went to my undergraduate at Indiana University, I had the fortune of taking a class by who I didn't know at the time would be my future doctoral supervisor, Kevin Hunt. He had run a chimpanzee field site at the Toro-Semliki Wildlife Reserve in Uganda since the mid-90s. I took all of his classes. I'd been a decent student in high school and college — like a B, B-plus average — but once I started taking these classes, the A's just got easy, because I was so compelled. The intrinsic motivation was just there: what is it about our sister species that can enlighten the human condition? Turns out, there's a lot. Now I teach a class called Sister Species: Lessons from the Chimpanzee.

What I was able to do in my dissertation was to go out to Uganda and study chimpanzee sleeping platforms in ecological context. I went out there with tree-climbing gear. If you've ever had an arborist fix a tree in your neighborhood, you've seen the gear they use. I learned how to do that haphazardly over a summer and a few lessons, and I went out to Uganda and started climbing these trees, sometimes 20, 25 meters in the air, well over 100 feet, to measure these chimpanzee sleeping platforms.

Because here's the thing: great apes are totally unique in that they build tree beds. We call them sleeping platforms. Jane Goodall originally called them nests, but they're not really nests, because a nest implies a fixed point, rearing young, long-term, multi-seasonal, you go back to. They're not this. The chimps make their bed every night. Only chimpanzees, bonobos, gorillas, and orangutans do this. And humans. So I thought, this seems like an interesting topic. There's not a lot being done on this work. This seems like a really cool dissertation project. Kevin thought the same.

It got me thinking, from an evolutionary standpoint: what is this chimp nest doing? What is this ape nest doing? Semliki was perfect, because there's a tree species there called Cynometra alexandri — Ugandan ironwood. It turns out humans love it as well, because it's very beautiful to work with when you're manufacturing furniture; it's very resilient hardwood. And it turns out the leafy components at the terminal ends of the branches — chimps love those too. They preferentially select it over 80% of the time, even though it's only about 5% of the local area. So in the mind of a chimp: oh, that's my bed, right there, I just need to make it now.

Over the course of my dissertation I did a series of studies, and discovered not only that the species is preferred, but why. It repels insects, especially Anopheles mosquitoes, which transmit malaria. It keeps you warm; all things being equal, it keeps you nicely thermoregulated. It keeps you away from macro-predators, because you're not sleeping on the ground, where in these environments there are lions, other hunting cats, snakes, big cobras. And it's this big, plush, beautiful sleep environment that potentially can help improve cognition.

For the second part of my dissertation, I went to work with Rob Shumaker at the Indianapolis Zoo, and we measured REM and non-REM in orangutans for the first time ever. We got this beautiful footage of an orangutan in REM. Then we did next-day cognitive tests, and it turns out, when you give them a big bed, they score better on these intrinsic-grammar touchscreen tests. So all of these things were things humans had to solve for when we went to the ground. We were leaving this beautiful, 18-million-year evolutionary tool up in the trees, and we had to come down. The answer to what happened next is the main thrust of the book.

So here's the answer to the sleep paradox. Around 1.5 to 1.8 million years ago — and I make a very thorough argument looking at the paleoanthropological record here — you see transformations in body size in Homo heidelbergensis. From the neck down, they're very much modern human; they just have half the brain size and their faces look a little different. They were doing a radical social experiment. They were kind of like the hippies of the primate order, because they were living in social group sizes that were very, very rare in primates: between 25 and 30 adults, in the shared project of reproduction and survival. They would go to different camps seasonally — very much starting to do what hunter-gatherers have done forever, and what hunter-gatherers like the Hadza, who I worked with, still do.

There's an acronym and a metaphor I use to describe this. I call it the human SHELL. This shell is effectively a mobile sleep exophenotype. That's a silly word, but it's a really cool word. It means that, just like a beaver creates a dam to alter its environment, and then that environment reshapes the genes of the beaver, humans did this with the sleep shell. When a beaver has an instinct, it makes the dam; the dam changes the environment, lowering the fluvial energy, lowering the water energy, creating little ponds right next to its home; and then, because it altered its environment, it becomes adapted, over evolutionary time, to hunting and foraging in those ponds. It created the conditions by which the environment is now shaping its genes. I believe humans did this with sleep.

So the SHELL stands for: Shelter, we started making acacia huts, places that minimize extremes in temperature. Hearth, because we innovated fire around one and a half million years ago, we had heat to control thermoregulatory conditions. Environmental preparation, moving seasonally to different places, creating crawls, altering conditions, picking the right cave, digging into it a little, creating crawl spaces. Lux control, once we had fire, for the first time we had 24-hour access to light, which opened up a whole new social niche, a hunting niche. But it's a special kind of light. It's not the light hitting our retinas right now, this cheap blue light. I call it fast-food light, because it's abundant, it's cheap, but it's nutritionally very poor. The red firelight allowed our ancestors to see, but never interfered with their melatonin production, the principal hormone that regulates sleep-wake activity. And then the last L — Lookouts. Because we had so many eyes, 24/7, around the camp, around the shell, we could go in and out of sleep flexibly.

So interestingly, the modern mind thinks of the social condition as one that reduces sleep. Oh, I've got to sleep alone, in this sleep cell, this prison, so that nothing interferes with my sleep. But ironically, it was the social evolution of our species that made us such high-efficiency sleepers. Weird and beautiful sleepers in the first place. That's the nutshell of the book, right there.

Ongoing thread. More from David R. Samson to follow.