On January 29th, 2018 we learned about

Everyone understands a lullaby: Human brains may have shaped universally-appreciated traits in music

Even if you don’t understand the lyrics, your brain may be wired to recognize the meaning of music from around the world. No matter what the finer nuances of a song may be, there’s evidence that some core element of a song’s genre may be universally understood. So just as you would never mistake a lullaby with a dance anthem in your native tongue, there’s a good chance you can also tell them apart when listening to songs by the Mentawai people of Indonesia. At least in broad strokes, it seems that humans everywhere have very similar concepts of what a dance versus a love song should sound like.

That idea may sound simple enough, but figuring out how to test is isn’t easy. Finding ways to untangle a person’s cultural background from music they’re listening to isn’t a small task, because that same background likely informs our perception of that music. To hunt for signs of universal musical concepts, researchers spent years assembling a database of music from around the world, then sorted that music by region and the type of song according to four categories: dance songs, lullabies, healing songs and love songs. Clips of those songs were played for English-speakers from 60 different countries, simply asking them to pick a category for each song they heard. If they could correctly identify a love song in a language and style they weren’t familiar with, it would suggest that some aspect of that song was shared across cultures.

With 750 listeners reporting, researchers found that dance songs and lullabies were the most easily identified. Apparently, soothing a baby is a similar process no matter where one lives. Love songs and healing songs weren’t so clear, a fact probably compounded by a lack of healing songs from Scandinavia or the United Kingdom. Overall, this first experiment suggests that there are some universally understood aspects of music, but figuring out what those are, and why they exist, are still open questions.

Possible sources of musical synchronicity

The hypothesis about how humans would end up with universal elements in our music range from ideas around our brain structure to a bit of convergent evolution. One explanation may be that some of these musical concepts, like a lullaby, are hard wired into our brains to be created and appreciated, with each culture adding their own flourish on top of a core formula for a song.

Alternatively, our brains may provide an interest in sound and stimuli, and that musicians have learned to trigger that response, with certain tunes just being the most time-tested method for doing so. In that case, a lullaby works because it takes advantage of brain functions that originally involved for other purposes, unrelated to music.

There’s also a chance that these shared traits between songs are a case of convergence. Through other environmental pressures, like a desire for community building, displaying status and more, musicians everywhere have been guided to similar-sounding songs. Each musical lineage started independently, rather than in any specific brain structure, but has nonetheless converged on a single way to perform a dance song because quicker tempos and more layered instrumentation just does that job better than anything else.

At this point, more people need to participate in the listening tests. While the initial participants hailed from many different countries, the second phase of testing will aim to include viewpoints of non-English speakers, particularly from more isolated cultures. If these songs are still universally understood at that point, we can really start looking for what defines each genre in our minds, and how that core evolved in the first place.

Source: Some Types Of Songs Are Universally Identifiable, Study Suggests by Rebecca Hersher, Goats and Soda

On December 10th, 2017 we learned about

Sorting through the spectrum of what chimpanzees regard as repulsive

You might not want to eat while reading this. According to a recent study that aimed to gross out chimpanzees, text probably isn’t enough to trigger the sense of disgust we’ve inherited from our ancestors. With that said, stories that bring up the issue of coprophagia, or eating feces, probably isn’t great for one’s appetite. At least not a first.

Digging into the details of disgust

Researchers were investigating where chimpanzees, as our closest genetic relative alive today, draw the line with what they’ll put in their mouths to eat. That line certainly wasn’t clear from the outset, as wild chimps will pick seeds out of poop to eat, and captive chimps will go a step further and snack on their poop outright. Researchers learned that there was some nuance to chimps’ consumption of crap, as the animals apparently evaluate feces based on genetic familiarity. A chimpanzee will eat their own poop, or that of closely related family members, but any other chimps’ waste elicits a clear display of disgust.

With that baseline established, researchers set up experiments to further probe chimpanzees’ criteria for when food is too revolting to eat. In one scenario, food was placed in an opaque box, either on top of soft but edible dough, or on top of a piece of rope. Chimps reaching in for the food were obviously repulsed by the soft, moist dough, yanking their hands out of the box as if it bit them. Other tests involved food being placed on what looked like feces, or near the scent of blood, and while no response was apparently quite as disgusting as something soft, wet and squishy, the chimps seemed to have similar guidelines for what was gross that humans do. They didn’t necessarily have the same standards though, as they would sometimes end up eating food from disgusting sources, but overall their criteria was pretty relatable.

Finding the value in what chimps find foul

The fact that these chimpanzees get grossed out like we do may seem obvious, but it wouldn’t have been safe to automatically assume they operated on similar criteria to humans. After all, any degree of coprophagia is probably too disgusting for humans to seriously consider, and researchers wanted to see exactly what our species had in common with our fellow primates. Avoiding substances, like poop or blood, that could easily harbor pathogens makes sense as a survival tactic, and identifying commonalities indicates that chimps and humans likely inherited some of these reference points from a shared ancestor. This work may also help zoos and conservationists manage the health of chimpanzees in their care. Dangerous substances can be presented in a more disgusting manner, and individual chimps that seem too casual about gross sources of food can be given extra attention for exposure to pathogens.

Source: What grosses out a chimpanzee? The origins of disgust by Kyoto University, EurekAlert!

On September 20th, 2017 we learned about

Experiments demonstrate how to manipulate monkey (and human) metacognition

For as many times as we tell our kids to believe in themselves, it’s good to keep in mind that confidence can sometimes be misleading. This isn’t to say that doubting your every decision is helpful or healthy, but that sometimes we don’t realize why we’re confident in the first place, opening us up to manipulation, such as putting more trust in a statement because it’s written in larger letters. This susceptibility isn’t exactly our fault though, as researchers have found that our primate cousins fall for the same tricks. While falling for these influences may seem like a drawback for us, it’s also proving that monkeys have a more sophisticated sense of self than they’re usually given credit for.

Understanding exactly what you do and do not know is called metacognition. It’s very helpful to know where your gaps in knowledge are so that you can adjust your actions accordingly. For instance, if you know you’ve never eaten a particular berry before, knowing you don’t know what it is will probably push you to investigate it more carefully before popping it into your mouth. This might seem obvious, but people, and apparently other primates, commonly make mistakes when evaluating our personal knowledge base, and that can obviously get us into trouble.

Confirming monkeys’ confidence

To test metacognition in monkeys, researchers had to train them on a multi-step game that would allow these non-verbal test subjects to demonstrate how much they thought they knew about something. Monkeys were presented with touch screens showing a single image, like a cricket, which they had to poke at to proceed. They then see that same cricket image again, along with three other images meant to distract them from their task, which is to poke the same cricket again.

After they make their selection, they’re shown a screen where they need to rate how confident they are about their previous poke. If they’re sure they’re right, they can pick an option that will net them three tokens for a correct answer, but cost them a token if they’re wrong. If they’re less sure about their answer, a low-confidence indicator will let them gain one token, even if they’re wrong. Monkeys are only rewarded once they earn a specific number of tokens, forcing them to play the long game to get a treat.

It’s a lot to throw at a monkey, but they seemed to get it enough to play and reveal patterns in their decision making. Once a monkey seemed to understand the mechanics of the game, researchers started manipulating how information was presented in order to manipulate confidence levels. For instance, higher-contrast images made the monkeys wager with more confidence, while low-contrast images had the opposite effect. These sorts of attributes change confidence levels in humans too, along with shorter, easier-to-pronounce vocabulary and the aforementioned larger text size.

Indecision versus imprecision

This isn’t to say that metacognition is just a form of self-delusion. Knowing when to take a shortcut, or react quickly and decisively, can be very helpful in certain scenarios. These traits probably evolved in a distant primate ancestor, and have been helping humans and monkeys for millions of years. Of course, it’s probably also helpful to know what you know versus what you think you know, since sometimes that same confidence can get you into trouble.

Source: Monkey sees. . . monkey knows? by Lindsey Valich, Rochester Newscenter

On September 4th, 2017 we learned about

Fossilized footprints found near Crete may seriously complicate our ancestors’ origins

Your foot may ache, smell and maybe need a toenail trim, but it’s a pretty special bit of anatomy. Even if you’re used to your single row of clawless of toes, it’s actually a unique arrangement among just about every animal on Earth. Our closest living relatives don’t have their hallux, or big toe, facing forward like we do, giving them a much more hand-shaped foot. These toes, along with the ball of our foot and long instep, come together to make a very distinct footprint. What’s exciting scientists now is that some of these prints have turned up in a time and place where they supposedly had no business being.

A set of footprints were found hardened into sedimentary rock on an island called Trachilos, off the coast of Crete. Based on the foraminifera, or marine microfossils, found in the layers of rock above and below the slab of stone in question, researchers confidently dated the prints as being 5.6 million years old. The catch is that at that time, no human ancestors, much less humans, were thought to be anywhere but Africa. What’s more, the shape of the prints look more like modern feet than any known ancestor living at that time.

Figuring out the who, what and where

Even without an actual bone or tool, these footprints may be enough to up-end our timeline for human evolution. The oldest confirmed hominid is Ardipithecus ramidus, who is thought to be a direct ancestor to modern humans. However, A. ramidus lived in Ethiopia around 4.4 million years ago, and at that point had a much more gorilla-shaped foot. We know that by 3.65 millions ago, our ancestor Australopithecus was leaving very modern-looking footprints in Tanzania, but neither of these dates sync up with the stroll some primate took through Crete at least a million years earlier.

Putting the evolutionary questions about foot-shapes aside, walking to the island of Trachilos is actually one of the easier issues to understand. North Africa and the Mediterrean Sea were very different places 5.6 million years ago, as the Sahara Desert was a savannah and the Mediterrean was beginning to dry up, thanks to the Strait of Gibraltar closing off the waters of the Atlantic Ocean. What’s more, Crete itself was still attached to the Greek mainland, making access to this particular plot of land all the more plausible.

Many questions to consider

Assuming this initial interpretation of these 50 footprints hold up, anthropologists have a lot of new questions to explore. Were the prints made by a hominid, or just a very similar gorilla living in Europe? Did this mystery primate walk out of Africa to Trachilos, having evolved our unique feet earlier than anyone thought? Were these prints left by a direct relative to modern Homo sapiens, or is this an offshoot of our family tree that didn’t end up succeeding with their early upright gait? Or, most radically, did our ancestors originate outside of Africa, strolling south instead of north? That last idea is a long-shot, and would still leave the species Homo sapiens as originating in Africa, but this simple stroll in the soft soil now demands that we investigate a lot of new possibilities.

My third grader said: I hope it turns out to be one of the crazier explanations!

Source: What Made These Footprints 5.7 Million Years Ago? by Gemma Tarlach, Dead Things

On August 29th, 2017 we learned about

Twisted and knotted cords found to tell tales beyond basic numerical tallies

234 years ago, Felipe Tupa Inka Yupanki arrived in the small Peruvian town of Collata to organize an uprising of native Incans against Spanish colonizers. Yupanki issued decrees and tried organizing an army, although the planned revolt came to a halt once these plans were discovered, ending in the rebel’s execution. This story isn’t as well known as the larger Incan rebellions of the eighteenth century, partially because it’s largely documented in collections of yarn and string in a writing medium called a khipu.

Recording words in twisted cords

Khipus are an Incan form of logosyllabic writing built out of twisted fibers and cords made from cotton, cloth or animal fibers like alpaca and llama hair. Shorter cords of variable color, patterning and textures are individually tied to one larger cord that acts like a sort of spine for the whole message. Each khipu is capped by a cayte, which was generally a textile like a ribbon or bandanna tied to the pieces creator, asserting its authenticity like a signature. If laid out flat, the whole khipu might resemble a short, uneven fringe, but that variability is key to their utility as a communication medium.

Many of the khipus now in museums have been found to use knots to record numbers, and were simple, portable accounting tools used by herders into the twentieth century. Unlike the beads of an abacus, knots were not directly representational of quantities, instead relying on distances and size to record different numbers. By and large, these kinds of accounting khipus were made of cotton, requiring less sophistication in their craftsmanship. However, two khipus recently studied in Collata display a new degree of depth and sophistication, showing that these objects could encode entire stories, as long as you know what to look for.

Narratives beyond numbers

The two khipus from Collata are known to reference the Incan rebellions, but they’re not really being read word for word at this point. Village elders have been passing the khipus, along with other documents tied to the town’s history, down to each other for generations, telling the story of the khipus in the process. The khipus are thought to be based on Quechua, a language no longer spoken in Collata. Still, the various twists, colors, and choices in fiber support the notion that this cords represent words, with 95 individual symbols having been identified.

Since my two sentence summary of the rebellion used 51 unique words, it’s easy to see that the khipu isn’t offering any one-to-one transcription. The 95 symbols that have been identified aren’t phonetic words, instead functioning more like rebuses or pictograms. So rather than directly represent a word, symbols that can remind someone of a word thanks to their similar sounds might be combined to express an idea. This system wouldn’t have been as flexible as a fully phonetic alphabet, but the 95 symbols seen across 487 cords on the Collata khipus are enough to have hundreds of unique combinations.

At this point, researchers speculate that these khipus may be a taste of a significant and unexplored communication system. The success of the Incan empire is hard to imagine without robust means of communication, and if more examples of narrative writing can be found, it may help unlock a lot of recorded knowledge about how that society functioned.

Source: Writing with Twisted Cords: The Inscriptive Capacity of Andean Khipus by Sabine Hyland, Current Anthropology

On August 28th, 2017 we learned about

Like humans, rhesus monkeys fixate on finding false faces in photos

In a time when people have a hard time seeing eye-to-eye, it may be comforting to know that we basically see the same world monkeys do. Studies of various monkeys’ perception have found that our fellow primates react to visual stimuli similarly to humans, even making the same mistakes we do. For instance, both humans and rhesus monkeys might confuse images of large, gray animals like elephants and rhinos, even though the monkeys lack contextual information to relate those creatures to each other. The visuals were perceived along the same underlying neural pathways, making an individual response from a monkey nearly indistinguishable from that of a human.

Now researchers have found a new shared “mistake” between rhesus monkeys and humans, which is called pareidolia, or seeing patterns like faces that aren’t really there. Monkeys were shown photos of inanimate objects that they’d have no real understanding of, such as coffee cups, purses and appliances. Photos that happened to have that critical two-dots-over-a-line pattern of a simple face held monkeys’ attention much longer than versions that didn’t. Based on previous data showing monkeys prefer to look at faces over non-faces, plus eye-tracking data that indicates the monkeys were examining objects’ “eyes” and “mouths” most of all, researchers are confident the monkeys are noting faces the same way we do. In fact, the when given the choice between a photo of a monkey’s face and an object’s faux-face, the object actually seemed to be the more interesting option for most test subjects.

Inherited interests

This fascination with faces isn’t surprising. Separate studies of macaque monkeys have found that the old world primates have the same functionality in their brain’s fusiform face area that we do. In that study, the monkeys recognized human faces as faces, despite our being a different species. Now that we know rhesus monkeys extend this activity to pareidolia like humans do, it’s increasingly likely that these socially-oriented specializations evolved in a distant, shared ancestor. Our interest in finding faces may not be a human trait after all, instead being shared among many primates that make a point to look into each other’s eyes.

Source: Rhesus monkeys found to see faces in inanimate objects too by Bob Yirka, Phys.org

On August 24th, 2017 we learned about

Holding someone’s hand can convince your brain to relax your cognitive load

I like holding my kids’ hands when we go places. My third grader is starting to assert some independence, and won’t casually hold hands as much as she used to, a fact that reminds me to appreciate my four-year-old’s tiny grip all the more. Scientists are finding that holding hands isn’t just a small pleasantry though, as it seems to trigger changes in participants’ brains that affect stress levels, cognition and even pain perception. In fact, it’s possible that part of my enjoyment in holding my child’s hand is that I’m offloading cognitive duties to them, leaving me with an easier stroll down the street.

Dr. Jim Coan has been investigating the effects of holding hands on the brain for years. Experiments generally involved pairs of people, one of which was in an fMRI machine to monitor brain activity. In each round of the experiment, the subjects would see either a red “X” or a blue “O” displayed on a screen, the former of which warned of a 20 percent chance of the person being scanned receiving a small electric shock 12 seconds later. During the 12 seconds after seeing a red “X,” most people’s brains showed a flurry of activity, from increases in stress to paying attention to the site of the possible pain. The big variable in all this was the partner’s touch.

Hands that help vs. those that hurt

Throughout these experiments, people would be asked to either hold hands or sit near each other. Holding hands with a trusted companion was found to make a huge difference in people’s reactions— there were fewer signs of stress, agitation and even pain all over the brain. In some cases, hypothalamus activity changed enough that it’s suspected to be part of the mechanism that makes people with social connections generally have better health than people who live alone.  A variation on the study had children with anxiety disorders hold the hands of their mothers while reading scary words like “monster” instead of receiving shocks, and that small bit of physical contact was soothing enough for the kids to behave as if anxiety wasn’t an issue.

This isn’t to say that holding hands is a cure-all. The above effects were only seen in cases where a person’s partner was someone they trusted and were connected to, such as a spouse, friend or reliable roommate. In variations where people being shocked while holding a stranger’s hand, the positive effects were nearly absent. For people who lived in areas with higher crime rates, strangers actually made things worse, strongly indicating that the physical contact of hand-holding isn’t as important as the social relationship between the people involved.

Social support as a starting point

This may seem intuitive, but that doesn’t explain why any of this happens. When researchers looked at areas of the brain like the prefrontal cortex, they expected to see that holding hands inhibited activity, like the comforting touch of a partner helped the brain tamp down worry and pain. They were wrong though, as no such “self control” could be detected. One clue to help reformulate their model for hand-holding was a variation on the shock experiments that found that threats to shock a partner triggered activity in the “safe” person’s brain as if they were in danger. In other other words, the brain treated a trusted partner almost like an extension of itself.

The new model for all this activity is that as highly social animals, humans actually treat having social contacts as our baseline, rather than a modifier. It’s not that holding hands is better than normal, it’s that sharing experiences with other people is normal, and suffering alone is the more difficult alternative. (In fact, people who have stronger preferences to work alone have also been found to have higher resting glucose levels, meaning they’ve got more energy of their own to expend on daily tasks.)

Shared safety net

So having a spouse or friend with you helps you basically relax a bit, sharing responsibility for well-being with that other person, a state accentuated when you’re physically connected. Why worry about every detail of a possible threat when your friend is there to assist you? In the case of my kids, I’m hopefully not offloading too much responsibility for our safety onto a four-year-old, although knowing that the kid is safe next to me certainly does help lower my stress levels in a parking lot. I’ll enjoy it while it lasts.

Source: Holding Hands is More Important Than You Think by Maximus Thaler, The Evolution Institute

On August 14th, 2017 we learned about

Humans are adept at appreciating alarm in the voices of animals

You’ve probably never conversed with a groundhog or tree frog, but it might not be as futile as you’d think. Sure, it’s sometimes hard to communicate with other humans that ostensibly speak the same language as you, but there’s a good chance that some of the underlying emotion you want to express gets through in just about any conversation. For all the layers of complexity that language can have, researchers are finding that humans are actually pretty decent at picking up the basic intent of a wide range of vertebrates’ vocalizations. We might not know exactly what details a particular prairie dog has to share, but we can at least tell how strongly that critter feels about what it’s saying.

Sensing species’ sentiment

The study was fairly straightforward, asking 75 humans to listen to recordings of different animals and identify the level of arousal, or emotional energy, of that animal. The humans were native speakers of English, German or Mandarin in order to try and eliminate any bias that might arise from a human language that somehow more closely followed the grammar rules of bush elephants or pigs. In the end, people’s assessments were definitely more accurate than random guesses, although the degree to which people understood each species was sometimes surprising.

Humans could identify higher or lower arousal in other humans quite well, followed by giant pandas, tree frogs, elephants and alligators. We actually did worse with some species pigs, ravens and barbary macaque monkeys, indicating that familiarity or genetic similarities weren’t the key component to communication. Overall, the more a species relied on shifting the frequency, or pitch, of their voice, the more it made sense to human ears. Some samples from the study can be heard here if you want to try it yourself.

Shared origins for animals’ outbursts

We know that animal vocalizations can get very complex, and so nobody was expecting anyone to really parse specific messages in this test. The fact that prairie dogs seem to have specific vocabulary for details in their alarm calls is probably going to be beyond the ear of most human listeners. However, the fact that the sense of urgency of an alarm call in prairie dogs, birds and other animals may be detectable may indicate that all these air-breathing vertebrates share a common foundation in our noise-making.

Even if you’re not about to chat with your local squirrels, this study helps establish aspects of how language may have evolved in the first place. Other work has found that specific vocalizations are often taught from one generation of animal to the next in a process that closely mirrors human language acquisition. For example, baby marmoset monkeys transition from babble to specific calls in a process that is nearly identical to human babies. A baby marmoset will get feedback from adults about specific phonemes it makes, and then learns to refine and rely on those specific sounds for communication as it matures.

Source: Humans identify emotions in voices of all air-breathing vertebrates

On July 20th, 2017 we learned about

Babies pick up second languages better when spoken to in “parentese”

Baby brains are language sponges. Among all lessons the world offers a tiny child, from gravity’s effect on spoons to how expressive adults can be about picking those spoons up, babies are also doing their best to make sense of everything people around them are saying. This is true even when more than one language is being spoken by the baby’s household, which is how most bilingual people pick up their first and second languages. Beyond the practical upside of being able to speak with a wider range of people in the world, being bilingual has been linked to a number of cognitive benefits. Since not everyone has a chance to grow up in a bilingual household, researchers have been looking at ways to help more children learn more languages.

Fortunately, while the ability to learn a language seems to be part of human genes, the specific language you learn is not. That bit of cultural information can be shared from anyone, and often babies will learn a second language from child care providers, schools, or the local population in general if parents speak a different language at home. There are also programs and classes, even for the very young, to help pick up a second language. As an experiment, a sort of minimal language course was established to help Spanish speakers in Spain learn English, and the results show that full immersion isn’t necessary for a child to make progress.

Learning languages in just one hour a week

The experimental class lasted 18 weeks, but it only met for one hour a week. During that hour, instructors catered to the type of interaction the participating 7- to 33-month-olds would receive at home. This meant that rather than more formal practice and instruction in English, the babies and toddlers were taught with something closer to the baby talk their parents were using at home to teach them Spanish. The idea was that those types of interactions aren’t just fun for the babies, but they’re actually something human brains have evolved to expect in order to learn to speak.

To evaluate the effectiveness of this “parentese” approach, children were outfitted with special vests that recorded their speech, both in the experimental class and a more traditional English class offered by the Madrid school system. Researchers then tallied up how many English words the kids used, and how often they used them. To see what stuck, their English was evaluated again 18 weeks after the instruction ended. The results showed that parentese worked better with these young kids, and their English was stronger at the end of 36 weeks, with those kids having retained around 74 words compared to the 18 words retained by more traditional students. Importantly, this held true even for kids from very different backgrounds— kids’ brains from both higher and lower income neighborhoods responded well to this type of instruction.

This doesn’t necessarily prove that these kids will all speak English fluently in a couple of years, but it does show that learning a second language doesn’t require intense investments of energy. Most likely, it means that the best way to teach a language is to use the tools evolution has created for a baby’s or toddler’s brain during this crucial developmental period when they’re primed to learn.

Source: New Study Shows How Exposure To A Foreign Language Ignites Infants’ Learning, Scienmag

On July 17th, 2017 we learned about

Grandparents’ earlier bedtimes may have evolved to ensure security for the family

Humans didn’t evolve in a world of deadbolts and heavy doors. Even a cave has a entrance that you might need to keep an eye on, but the average family or village probably didn’t have a sentry staying up to look for lions. Instead, security was likely assured thanks to restless grandparents and the fact that older people’s sleep schedules rarely sync up with those of younger adults.

Duke researchers built this “poorly sleeping grandparent hypothesis” around sleep data from the Hazda people of northern Tanzania. Hazda villagers will often work separately during the day, then come together to sleep at night in groups of 20 to 30 people. With no artificial lighting and simple bedding laid out on the ground near and outdoor hearth, the Hazda are thought to be a good proxy for early human societies, possibly providing insight into how our sleep needs evolved long ago. To get the details on this lifestyle, study participants were asked to wear activity monitors so that each individual’s schedule could be compared.

Sleeping in unscheduled-shifts

The pattern that emerged is likely very familiar to many of us, even if we’re not used to sleeping outdoors. Older people tended to go to bed earlier, around 8:00 pm, while younger adults stayed up later. Appropriately, older people also woke up earlier as well, but that doesn’t fully explain how this kind of sleep schedule could be a benefit to a group’s security. The other element is how restless people were during the the night— at any given moment during the night, at a little over a third of the group was dozing or alert. There were only 18 minutes in a night when every adult was completely at rest.

This suggests that a group of mixed ages could sleep without sentries because everyone was “on duty” for part of the night. The fact that older people tend to sleep and wake early may have then evolved to reinforce this system. With sleep schedules staggered by a few hours, an extended family would have very little risk of being truly unguarded throughout the night. This is all without factoring the further disruptions of young children, who would presumably add another layer of schedules to the mix (while creating new demands of their own that a grandparent might be able to help with.)

Interestingly, these overlapping but different schedules not only avoided lion-surprises, but also provided sufficient rest. Nobody complained of sleep deprivation, suggesting that while we obviously need good amounts of sleep, the idea that a good night’s rest has to be a single, uninterrupted block of time may be a modern expectation.

Source: Live-in grandparents helped human ancestors get a safer night's sleep, Popular Archaeology