Are humans the only species to suffer from impacted teeth?

Are humans the only species to suffer from impacted teeth?

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Many humans need their third molars (wisdom teeth) removed due to tooth impaction. Are humans the only species to suffer from this?

According to Wikipedia An impacted tooth is one that fails to erupt into the dental arch within the expected developmental window. Because impacted teeth do not erupt, they are retained throughout the individual's lifetime unless extracted or exposed surgically.

Now to your interesting question, there are case reports, that yes, animals, other than humans can also have impacted teeth.

  1. Mandibular canine tooth impaction in young dog which was then made to erupt by surgery. Reference

  2. Maxillary canine impaction in a persion cat. Reference

According to this article

Proper growth and development of the oral cavity depends on a series of events that must occur normally and in the proper sequence. Genetic abnormalities or trauma that affects either the developing tissues or the timing of their development can cause abnormalities. Defects that decrease comfort, health, or function require treatment; those that result in only an esthetic problem do not. Common developmental problems include persistent deciduous teeth, unerupted teeth, malformed teeth, malocclusion, and malformed jaws.

We're the Only Animals With Chins, and No One Knows Why

“Little pig, little pig, let me come in,” says the big, bad wolf. “No, no, not by the hair on my chinny chin chin,” say the three little pigs. This scene is deeply unrealistic and not just because of the pigs' architectural competence, the wolf's implausible lung capacity, and everyone's ability to talk.

The thing is: Pigs don't have chins. Nor do any animals, except for us.

The lower jaw of a chimpanzee or gorilla slopes backwards from the front teeth. So did the jaw of other hominids like Homo erectus. Even Neanderthal jaws ended in a flat vertical plane. Only in modern humans does the lower jaw end in a protruding strut of bone. A sticky-outy bit. A chin.

“It's really strange that only humans have chins,” says James Pampush from Duke University. “When we're looking at things that are uniquely human, we can't look to big brains or bipedalism because our extinct relatives had those. But they didn't have chins. That makes this immediately relevant to everyone.” Indeed, except in rare cases involving birth defects, everyone has chins. Sure, some people have less pronounced ones than others, perhaps because their lower jaws are small or they have more flesh around the area. But if you peeled back that flesh and exposed their jawbones—and maybe don't do that—you'd still see a chin.

There are no firm answers, which isn't for lack of effort. Evolutionary biologists have been proposing hypotheses for more than a century, and Pampush has recently reviewed all the major ideas, together with David Daegling. “We kept showing, for one reason or another, that these hypotheses are not very good,” he says.

Read Follow-Up Notes

The most heavily promoted explanation is that chins are adaptations for chewing—that they help to reduce the physical stresses acting upon a masticating jaw. But Pampush found that, if anything, the chin makes things worse. The lower jaw consists of two halves that are joined in the middle when we chew, we compress the bone on the outer face of this join (near the lips) and pull on the bone on the inner face (near the tongue). Since bone is much stronger when compressed than pulled, you'd ideally want to reinforce the inner face of the join and not the outer one. In other words, you'd want the opposite of a chin.

Others have suggested that the chin is an adaptation for chinwags: It resists the forces we create when speaking. After all, speech is certainly a feature that separates us from other living animals. But there's no good evidence that the tongue exerts substantial enough forces to warrant a thick chunk of reinforcing bone. “And any mammal that also communicates vocally or suckles or engages in complex feeding behaviors that involve the tongue are probably experiencing similar stresses and strains, and they're not getting chins,” says Pampush.

Maybe it's about sex, then? Men typically have bigger chins than women, and stronger chins are often equated with attractiveness. Perhaps the chin is a sexual ornament, the human equivalent of a stag's antlers or a peacock's tail, a way of attracting mates or perhaps even signaling one's health and quality. “But if that's the case, we'd be the only mammal ever where both sexes have selected for the exact same ornament,” says Pampush. In other words, women have chins, too. Chin shape may well be relevant to sex, but that doesn't explain chin presence. “They must have been there for some other reason before we started looking at the shape of them.”

Then, there are hypotheses that “stretch the concept of natural selection,” says Pampush. For example, one century-old idea says that chins are adaptations for deflecting punches to the face. That is, they helped early humans to take one on the chin. “That would require humans to hit each other so often, and to suffer such dire consequences from being hit without a chin . it's unrealistic,” says Pampush. Also, chins are terrible for deflecting blows. They don't disperse the incoming forces very evenly, which results in broken jaws. Even if our ancestors were constantly pummeling each other in the face, they would have fared better by reinforcing their jaws all the way round.

Pampush doubts that chins are adaptations at all. He thinks it's more likely that they are spandrels—incidental features that have no benefits in themselves, but are byproducts of evolution acting upon something else.

For example, during human evolution, our faces shortened and our posture straightened. These changes made our mouths more cramped. To give our tongues and soft tissues more room, and to avoid constricting our airways, the lower jaw developed a forward slope, of which the chin was a side effect. The problem with this idea is that the chin's outer face doesn't follow the contours of its inner face, and has an exceptionally thick knob of bone. None of that screams “space-saving measure.”

A different explanation portrays the chin as a bit of the jaw that got left behind while the rest shrunk back. As early humans started cooking and processing our food, we made fewer demands upon our teeth, which started shrinking as a result. They gradually retracted into the face, while the part of the lower jaw that held them did not (or, at least, did so more slowly). Hence: chin.

Stephen Jay Gould and Richard Lewontin, who coined the concept of evolutionary spandrels, liked this hypothesis. So does Nathan Holton from the University of Iowa, who studies facial evolution. “It seems that the appearance of the chin itself is probably related to patterns of facial reduction in humans during the Pleistocene,” he says. “In this sense, understanding why faces became smaller is important to explaining why we have chins.”

“But why did the lower border of the jaw also not shrink?” Pampush asks. “What happened that left that last little bit sticking out?” This is the problem with spandrel hypotheses more generally: They're often very hard to test.

It may seem frustrating to have so many imperfect competing hypotheses, but that's part of the joy of chins: They reveal something about how scientists think about evolution. Some see the sculpting power of natural selection in everything, and view chins as surely some kind of adaptation. Others see natural selection as just one of many evolutionary forces, and so gravitate towards a spandrel-based explanation. “The chin is one of these rare phenomena in evolutionary biology that really exposes the deep philosophical differences between researchers in the field,” says Pampush.

And, indeed, between people outside the field. “I always get entertaining emails from lay people trying to help me so let me thank you in advance for what I'm about to receive,” he tells me.

Because if there's one trait that more universally human than the chin, it's having opinions.

The Top Ten Daily Consequences of Having Evolved

Natural selection acts by winnowing the individuals of each generation, sometimes clumsily, as old parts and genes are co-opted for new roles. As a result, all species inhabit bodies imperfect for the lives they live. Our own bodies are worse off than most simply because of the many differences between the wilderness in which we evolved and the modern world in which we live. We feel the consequences every day. Here are ten.

 1. Our cells are weird chimeras
Perhaps a billion years ago, a single-celled organism arose that would ultimately give rise to all of the plants and animals on Earth, including us. This ancestor was the result of a merging: one cell swallowed, imperfectly, another cell. The predator provided the outsides, the nucleus and most of the rest of the chimera. The prey became the mitochondrion, the cellular organ that produces energy. Most of the time, this ancient symbiosis proceeds amicably. But every so often, our mitochondria and their surrounding cells fight. The result is diseases, such as mitochondrial myopathies (a range of muscle diseases) or Leigh’s disease (which affects the central nervous system).

2. Hiccups
The first air-breathing fish and amphibians extracted oxygen using gills when in the water and primitive lungs when on land—and to do so, they had to be able to close the glottis, or entryway to the lungs, when underwater. Importantly, the entryway (or glottis) to the lungs could be closed. When underwater, the animals pushed water past their gills while simultaneously pushing the glottis down. We descendants of these animals were left with vestiges of their history, including the hiccup. In hiccupping, we use ancient muscles to quickly close the glottis while sucking in (albeit air, not water). Hiccups no longer serve a function, but they persist without causing us harm—aside from frustration and occasional embarrassment. One of the reasons it is so difficult to stop hiccupping is that the entire process is controlled by a part of our brain that evolved long before consciousness, and so try as you might, you cannot think hiccups away.

3. Backaches
The backs of vertebrates evolved as a kind of horizontal pole under which guts were slung. It was arched in the way a bridge might be arched, to support weight. Then, for reasons anthropologists debate long into the night, our hominid ancestors stood upright, which was the bodily equivalent of tipping a bridge on end. Standing on hind legs offered advantages—seeing long distances, for one, or freeing the hands to do other things—but it also turned our backs from an arched bridge to an S shape. The letter S, for all its beauty, is not meant to support weight and so our backs fail, consistently and painfully.

4. Unsupported intestines
Once we stood upright, our intestines hung down instead of being cradled by our stomach muscles. In this new position, our innards were not as well supported as they had been in our quadrupedal ancestors. The guts sat atop a hodgepodge of internal parts, including, in men, the cavities in the body wall through which the scrotum and its nerves descend during the first year of life. Every so often, our intestines find their way through these holes—in the way that noodles sneak out of a sieve—forming an inguinal hernia.

5. Choking
In most animals, the trachea (the passage for air) and the esophagus (the passage for food) are oriented such that the esophagus is below the trachea. In a cat's throat, for example, the two tubes run roughly horizontal and parallel to each other before heading on to the stomach and lung, respectively. In this configuration, gravity tends to push food down toward the lower esophagus. Not so in humans. Modifications of the trachea to allow speech pushed the trachea and esophagus further down the throat to make way. Simultaneously, our upright posture put the trachea and esophagus in a near-vertical orientation. Together these changes leave falling food or water about a 50-50 chance of falling in the “wrong tube.” As a consequence, in those moments in which the epiglottis does not have time to cover the trachea, we choke. We might be said to choke on our success. Monkeys suffer the same fate only rarely, but then again they can’t sing or dance. Then again, neither can I.

6. We're awfully cold in winter
Fur is a warm hug on a cold day, useful and nearly ubiquitous among mammals. But we and a few other species, such as naked mole rats, lost it when we lived in tropical environments. Debate remains as to why this happened, but the most plausible explanation is that when modern humans began to live in larger groups, our hair filled with more and more ticks and lice. Individuals with less hair were perhaps less likely to get parasite-borne diseases. Being hairless in Africa was not so bad, but once we moved into Arctic lands, it had real drawbacks. Evolution has no foresight, no sense of where its work will go.

7. Goosebumps don't really help
When our ancestors were covered in fur, muscles in their skin called “arrector pili” contracted when they were upset or cold, making their fur stand on end. When an angry or frightened dog barks at you, these are the muscles that raise its bristling hair. The same muscles puff up the feathers of birds and the fur of mammals on cold days to help keep them warm. Although we no longer have fur, we still have fur muscles just beneath our skin. They flex each time we are scared by a bristling dog or chilled by a wind, and in doing so give us goose bumps that make our thin hair stand uselessly on end.

8. Our brains squeeze our teeth
A genetic mutation in our recent ancestors caused their descendants to have roomy skulls that accommodated larger brains. This may seem like pure success—brilliance, or its antecedent anyway. But the gene that made way for a larger brain did so by diverting bone away from our jaws, which caused them to become thinner and smaller. With smaller jaws, we could not eat tough food as easily as our thicker-jawed ancestors, but we could think our way out of that problem with the use of fire and stone tools. Yet because our teeth are roughly the same size as they have long been, our shrinking jaws don’t leave enough room for them in our mouths. Our wisdom teeth need to be pulled because our brains are too big.

9. Obesity
Many of the ways in which our bodies fail have to do with very recent changes, changes in how we use our bodies and structure our societies. Hunger evolved as a trigger to drive us to search out food. Our taste buds evolved to encourage us to choose foods that benefited our bodies (such as sugar, salt and fat) and avoid those that might be poisonous. In much of the modern world, we have more food than we require, but our hunger and cravings continue. They are a bodily GPS unit that insists on taking us where we no longer need to go. Our taste buds ask for more sugar, salt and fat, and we obey.

10 to 100. The list goes on.
I have not even mentioned male nipples. I have said nothing of the blind spot in our eyes. Nor of the muscles some of use to wiggle our ears. We are full of the accumulated baggage of our idiosyncratic histories. The body is built on an old form, out of parts that once did very different things. So take a moment to pause and sit on your coccyx, the bone that was once a tail. Roll your ankles, each of which once connected a hind leg to a paw. Revel not in who you are but who you were. It is, after all, amazing what evolution has made out of bits and pieces. Nor are we in any way alone or unique. Each plant, animal and fungus carries its own consequences of life's improvisational genius. So, long live the chimeras. In the meantime, if you will excuse me, I am going to rest my back.

Editor's note: A previous version of this article stated that your ankles once connected a foreleg to a paw. This version has been corrected to say hind leg.

From skeletons to teeth, early human fossils have been found of more than 6,000 individuals. With the rapid pace of new discoveries every year, this impressive sample means that even though some early human species are only represented by one or a few fossils, others are represented by thousands of fossils. From them, we can understand things like:

  • how well adapted an early human species was for walking upright
  • how well adapted an early human species was for living in hot, tropical habitats or cold, temperate environments
  • the difference between male and female body size, which correlates to aspects of social behavior
  • how quickly or slowly children of early human species grew up.

While people used to think that there was a single line of human species, with one evolving after the other in an inevitable march towards modern humans, we now know this is not the case. Like most other mammals, we are part of a large and diverse family tree. Fossil discoveries show that the human family tree has many more branches and deeper roots than we knew about even a couple of decades ago. In fact, the number of branches our evolutionary tree, and also the length of time, has nearly doubled since the famed ‘Lucy’ fossil skeleton was discovered in 1974!

There were periods in the past when three or four early human species lived at the same time, even in the same place. We – Homo sapiens – are now the sole surviving species in this once diverse family tree.

While the existence of a human evolutionary family tree is not in question, its size and shape - the number of branches representing different genera and species, and the connections among them – are much debated by researchers and further confounded by a fossil record that only offers fragmented look at the ancient past. The debates are sometimes perceived as uncertainty about evolution, but that is far from the case. The debates concern the precise evolutionary relationships - essentially, ‘who is related to whom, and how.’ Click here to explore information about different early human species.

What is the latest theory of why humans lost their body hair? Why are we the only hairless primate?

We humans are conspicuous among the 5,000 or so mammal species in that we are effectively naked. Just consider what your pet dog or cat (or, for that matter, a polar bear) would look like, and how it might feel, if its furry coat were shorn.

Scientists have suggested three main explanations for why humans lack fur. All revolve around the idea that it may have been advantageous for our evolving lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.

The aquatic-ape hypothesis suggests that six million to eight million years ago apelike ancestors of modern humans had a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is&mdashand helpful in providing us with an excuse for being overweight&mdashpaleontological evidence for an aquatic phase of human existence has proven elusive.

The second theory is that we lost our fur in order to control our body temperature when we adapted to life on the hot savannah. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. The body-cooling idea seems sensible, but even though lacking fur might have made it easier for us to lose heat during the day, we also would have lost more heat at night, when we needed to retain it.

Recently, a colleague and I suggested that ancestors to modern humans became naked as a means to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other "ectoparasites." These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.

Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling "overcooked sausages with buck teeth," also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.

Once hairlessness had evolved this way, it may have become subject to sexual selection&mdashbeing a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock's tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.

“You are what you eat.” Can these pithy words explain the evolution of the human species?

Yes, says Richard Wrangham of Harvard University, who argues in a new book that the invention of cooking — even more than agriculture, the eating of meat, or the advent of tools — is what led to the rise of humanity.

Wrangham’s book “Catching Fire: How Cooking Made Us Human” is published today by Basic Books. In it, he makes the case that the ability to harness fire and cook food allowed the brain to grow and the digestive tract to shrink, giving rise to our ancestor Homo erectus some 1.8 million years ago.

“Cooking is the signature feature of the human diet, and indeed, of human life — but we have no idea why,” says Wrangham, the Ruth Moore Professor of Biological Anthropology in Harvard’s Faculty of Arts and Sciences. “It’s the development that underpins many other changes that have made humans so distinct from other species.”

Drawing on a wide body of research, Wrangham makes the case that cooking makes eating faster and easier, and wrings more caloric benefit from food. Moreover, he writes, cooking is vitally important to supporting the outsize human brain, which consumes a quarter of the body’s energy.

By freeing humans from having to spend half the day chewing tough raw food — as most of our primate relatives do — cooking allowed early humans to devote themselves to more productive activities, ultimately allowing the development of tools, agriculture, and social networks. Cooked food is also softer, meaning the body uses less energy digesting what it takes in.

Since physical remnants of fire tend to degrade rapidly, archaeological evidence of fire and cooking dates back only about 800,000 years. Wrangham looked to biological evidence, which shows that around 1.8 million years ago, Homo erectus arose with larger brains and bodies and smaller guts, jaws, and teeth — changes consistent with the switch to a more tender and energetically rich diet of cooked food.

“Cooking is what makes the human diet ‘human,’ and the most logical explanation for the advances in brain and body size over our ape ancestors,” Wrangham says. “It’s hard to imagine the leap to Homo erectus without cooking’s nutritional benefits.”

While others have posited that meat-eating enabled the rise of Homo erectus some 1.8 million years ago, Wrangham says those theories don’t mesh with that species’ smaller jaws and teeth. Instead, he claims meat enabled the shift from australopithecines to Homo habilis — a species about the size of a chimp, but with a bigger brain — more than half a million years earlier.

Wrangham says the adoption of cooking had profound impacts on human families and relationships, making hearth and home central to humanity and driving humans into paired mating and perhaps even traditional male-female household roles.

He writes that the advent of cooking permitted a new distribution of labor between men and women: Men entered into relationships to have someone to cook for them, freeing them up for socializing and other pursuits and bolstering their social standing. Women benefited from men’s protection, safeguarding their food from thieves. Homo sapiens remains the only species in which theft of food is uncommon even when it would be easy.

“To this day, cooking continues in every known human society,” Wrangham says. “We are biologically adapted to cook food. It’s part of who we are and affects us in every way you can imagine: biologically, anatomically, socially.”

Public concern on human health impact of plastic pollution

The impact of marine plastic pollution on human health tops a list of health-related concerns over marine threats in a large scale survey which could help shape policy over how best to protect our oceans.

Researchers at the University of Exeter led a survey of more than 15,000 people across 14 European countries, plus Australia, as part of the interdisciplinary European collaboration called the Seas, Oceans and Public Health in Europe (SOPHIE) Project, funded by Horizons 2020.

Working with colleagues from the European Marine Board, the University of Vienna and the University of Queensland, the SOPHIE project investigated public perceptions towards various marine topics, including marine plastic pollution. The new study, published in Global Environmental Change, found that both Europeans and Australians were highly concerned about the human health impact of marine plastic pollution, ranking it top of 16 marine-related threats in terms of cause for concern, including chemical or oil spills, marine biodiversity loss and climate change related effects such as sea-level rise and ocean acidification.

The research comes as plastic pollution is widely acknowledged as a major cause for international concern. Tiny particles of plastic known as microplastic have been found in all sea life sampled, meaning they are likely to be ingested by humans. However, while much is known about the ecological damage, including to marine life and other wildlife, the potential impacts on human health are inconclusive. The study found that people surveyed supported more research to understand the impact of marine plastic pollution on our health.

Lead author Sophie Davison, of the University of Exeter's European Centre for Environment and Human Health, said: "Plastic pollution is one of the fastest-growing environmental challenges on our planet. Yet, while the damage to marine life is well understood, the impact on human health remains unclear. Our study indicates that this is of grave concern to the public, and that there's widespread support for more research in this area."

Research has shown that plastic pollution breaks down to miniscule particles of microplastic, which find their way into the guts of sea creatures, birds and other wildlife. Yet to date, the evidence surrounding if and how they affect humans, for example by ingesting them through eating seafood, is limited.

Co-author Mathew White, an environmental psychologist at the University of Vienna, said the paper aimed to inform decision-making around policy on plastic pollution and funding for research into potential human health impacts. He said: "Given that marine plastic pollution is a global challenge and all of society contributes to some degree to the plastic consumption cycle, we urgently need to find ways of connecting the high level of concern with ways of curbing the leakage of plastic into the environment."

The findings echo a recent poll of 8,000 people, conducted by the Government's Department for the environment, food and rural Affairs. The survey found that three quarters of respondents felt that plastic pollution and litter was the greatest threat to the health of the seas, and 94 per cent of people believe the health of oceans and humans are inextricably linked, in turn echoing a warning from researchers led by Exeter which set out an action plan to instigate the first stages of change.

The University of Exeter is a world leader on microplastics research, including the biological impact on marine animals, and developing a new method to test for different types of plastic simultaneously.

Diet and Health

While the marine diet of ancient Croations is exciting news for scientists, other finds have proven just as spectacular. Take, for example, the Australopithecine Lucy. Chemical analysis of her teeth shows that, as far back as 4 million years ago, the diets of hominins suddenly became much more diversified than other primates. Apes living in trees were still ordering off the prix-fixe menu of the jungle, whereas the more human-like hominins had expanded their palate to the buffet offerings of jungle and savannah.

Carbon signatures of the ancient teeth show that Lucy and her kin had expanded beyond fruits and soft buds of trees and shrubs to actually eating other animals the development of thicker enamel reflects that they had also developed more protection to eat seeds, nuts and roots. “To what extent this dietary shift reflected active hunting, or the gathering of small prey such as arthropods, or scavenging, or a combination of all these, is still unclear, but something plainly happened,” writes paleoanthropologist Ian Tattersall in The Strange Case of the Rickety Cossack and Other Cautionary Tales from Human Evolution.

The dental record can also reveal important markers of health to anthropologists. In one study, researchers looked at dentine—the tissue that forms beneath enamel—in modern Greek people, compared to prehistoric Middle Eastern communities. Modern Greeks had levels of vitamin D deficiency that were four times higher than their ancient ancestors, perhaps due to spending more time indoors or changes in clothing, though researchers have yet to find a definitive answer. Pre-agriculture peoples also had significantly lower rates of cavities, and researchers have begun extracting bacterial DNA from calcified plaque to see how strains of bacteria changed after the introduction of farming.

Despite claims made by adherents of the "Paleo Diet" (which, to be clear, is not reflective of an actual paleolithic diet), not all health outcomes of prehistoric life were positive. Debbie Guatelli-Steinberg, an anthropologist at the Ohio State University and author of What Teeth Reveal About Human Evolution, has seen firsthand how disease and malnutrition plagued Neanderthals. For this she studies linear markings on the enamel called hypoplasias, which occur when enamel formation stops for a short period due to genetic causes or environmental ones.

“Some disruptions [in Neanderthals] were quite long, almost up to three months,” Guatelli-Steinberg says of her research. “It’s difficult to interpret, but when there’s a long period of time like that, it might be more likely that it has something to do with malnutrition.”

Lower teeth recovered from a cave in Southern China provided evidence of the earliest unequivocal modern humans in the region. (Liu Wu et al / Nature)

The Braided Stream

When a major new find is made in human evolution—or even a minor new find—it’s common to claim it overturns all previous notions of our ancestry. Perhaps having learned from past mistakes, Berger doesn’t make such assertions for Homo naledi—at least not yet, with its place in time uncertain. He doesn’t claim he has found the earliest Homo, or that his fossils return the title of “Cradle of Humankind” from East to South Africa. The fossils do suggest, however, that both regions, and everywhere in between, may harbor clues to a story that is more complicated than the metaphor “human family tree” would suggest.

“What naledi says to me is that you may think the record is complete enough to make up stories, and it’s not,” said Stony Brook’s Fred Grine. Maybe early species of Homo emerged in South Africa and then moved up to East Africa. “Or maybe it’s the other way around.”

Berger himself thinks the right metaphor for human evolution, instead of a tree branching from a single root, is a braided stream: a river that divides into channels, only to merge again downstream. Similarly, the various hominin types that inhabited the landscapes of Africa must at some point have diverged from a common ancestor. But then farther down the river of time they may have coalesced again, so that we, at the river’s mouth, carry in us today a bit of East Africa, a bit of South Africa, and a whole lot of history we have no notion of whatsoever. Because one thing is for sure: If we learned about a completely new form of hominin only because a couple of cavers were skinny enough to fit through a crack in a well-explored South African cave, we really don’t have a clue what else might be out there.