The unique human body part that evolution cannot explain
The human body is a machine whose many parts – from the microscopic details of our cells to our limbs, eyes, liver and brain – have been assembled in fits and starts over the four billion years of our history.
But scientists are still puzzling over why we evolved into this particular form. Why do humans uniquely have a chin, for example? And why, relative to body weight, is a human testicle triple the size of a gorilla's but a fifth of that of a chimpanzee? As I show in my new book, The Tree of Life, we are still searching for the answers to many of these 'why' questions. But we are starting to find answers to some of them.
The story of evolution tells us how, starting from simple beginnings, each species was built, when each of the components that make a living creature was added to its blueprint. If we climb the evolutionary tree of life, we can follow a twisting path that visits the increasingly specialised branches that a species belongs to. We humans, for example, were animals before we became vertebrates; mammals before evolving into primates and so on.
The groups of species we share each of these branches with reveal the order our body parts appeared.
A body and a gut (inventions of the animal branch) must have come before backbone and limbs (vertebrate branch); milk and hair (mammals) came before fingernails (primates).
There is a way we can study the separate problem of just why we evolved each of these body parts, but it only works if the feature in question has evolved more than once on separate branches of the tree of life. This repeated evolution is called convergence. It can be a source of frustration for biologists because it confuses us as to how species are related. Swallows and swifts, for example, were once classified as sister species. We now know from both DNA and comparisons of their skeletons that swallows are really closer relatives of owls than swifts.
Size matters when it comes to evolution
But convergent evolution becomes something useful when we think of it as a kind of natural experiment. The size of primate testicles gives us a classic example. Abyssinian black and white colobus monkey and bonnet macaque adult males are roughly the same size. But, like chimps, humans and gorillas, these similar monkeys have vastly dissimilar testicles. Colobus testicles weigh just 3 grams. The testicles of the macaques, in contrast, are a whopping 48 grams.
You could come up with several believable explanations for their different testicle sizes. Large testicles might be the equivalent of the peacock's tail, not useful per se but attractive to females. But perhaps the most plausible explanation relates to the way they mate. A male colobus monkey competes ferociously for access to a harem of females who will mate exclusively with him. Macaques, on the other hand, live in peaceful mixed troops of about 30 monkeys and have a different approach to love where everyone mates with everyone else: males with multiple females (polygamy) and females with multiple males (polyandry).
The colobus with his harem can get away with producing a bare minimum of sperm – if a droplet is enough to produce a baby, then why make more? For a male macaque, the competition to reproduce happens in a battle between his sperm and the sperm of other males who mated before or after. A male macaque with large testicles should make more sperm, giving him a higher chance of passing on his genes. It's a sensible explanation for their different testicle sizes, but is it true? This is where convergent evolution helps.
If we look across the whole of the mammal branch of the tree of life, we find there are many groups of mammals that have evolved testicles of all different sizes. In almost all these separate cases, larger testicles are consistently found in promiscuous species and smaller in monogamous.
A small-testicled, silverback male gorilla has sole access to a harem. Big-testicled chimps and bonobos are indeed highly promiscuous. Dolphins, meanwhile, may have the biggest mammalian testicles of all, making up as much as 4 per cent of their body weight (equivalent to human testicles weighing roughly 3 kilos). Although wild dolphin sex lives are naturally hard to study, spinner dolphins at least fit our expectations, engaging in mass mating events called wuzzles.
It was thanks to the multiple observations provided by convergent evolution that we were able to discover this consistent correlation between testicle size and sex life right across the mammals. And as for humans, we have testicle size somewhere in the middle, you can make of this what you want!
But what of the human chin?
The human chin has been fertile ground for arguments between scientists over its purpose. As with testicles, there are half a dozen plausible ideas to explain the evolution of the human chin. It could have evolved to strengthen the jaw of a battling caveman. Maybe the chin evolved to exaggerate the magnificence of a manly beard. It might even be a by-product of the invention of cooking and the softer food it produced – a functionless facial promontory left behind by the receding tide of a weakening jaw.
Intriguingly, however, a chin can be found in no other mammal, not even our closest cousins, the Neanderthals. Thanks to the uniqueness of the homo sapiens chin, while we have a rich set of possible explanations for its evolutionary purpose, in the absence of convergent evolution, we have no sensible way of testing them.
Some parts of human nature may be destined to remain a mystery.
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Daily Mail
an hour ago
- Daily Mail
Look familiar? Scientists reveal what Neanderthals and Denisovans would look like TODAY if they hadn't gone extinct
For the last 40,000 years, Homo sapiens have been the only human species walking the Earth. Our ancient ancestors died out thousands of years ago, leaving behind nothing but fossils, a few scattered artefacts, and lingering traces in our DNA. But what if things had turned out differently? MailOnline has asked the experts to find out what the world might look like if the Neanderthals and Denisovans hadn't gone extinct. Surprisingly, they say that our distant evolutionary cousins might not be all that different to modern humans today. However, they might have had a hard time fitting in with our fast-paced, highly social societies. Dr April Noel, a palaeolithic archaeologist from the University of Victoria, told MailOnline: 'The idea that Neanderthals were hunched over, dim-witted individuals with no thought beyond their next meal is no longer tenable. 'At the same time, the idea that you could just slap a hat on a Neanderthal and you would not think twice about sitting next to him on the tube is also out the window.' What would they look like? Neanderthals and Denisovans are our closest ancient human relatives. The Neanderthals emerged around 400,000 years ago when they branched off from our common ancestors. Denisovans, meanwhile, are a far more elusive species of ancient humans who split from the Neanderthal evolutionary line around 430,000 years ago. If they had remained as separate species rather than going extinct, Neanderthals and Denisovans might look much the same as they did in the distant past. From the abundant fossil records, we know that Neanderthals were a little shorter than us on average, with shorter legs and wider hips. Neanderthals were very muscular and rugged, with large bodies and even larger heads. Their skulls show that they have room for a bigger brain than modern humans and would have been distinguished by a massive brow ridge and small foreheads. Neanderthals are our closest living relatives and share all our features to some degree. However, neanderthals have a stronger brow and a smaller forehead. Their skin tone would have depended on their climate, much like modern humans today (AI-generated impression) What would Neanderthals look like today? If Neanderthals survived until today, they might keep many of their original traits. This means they would be stockier and more heavily built than modern humans, with shorter legs and larger heads. Their faces would be distinguished by heavy brows and small foreheads. However, experts say that humans and Neanderthals would probably keep interbreeding. This means that these traits would become mixed with those of Homo sapiens. However, experts say they still would be clearly recognisable as fellow humans. Professor John Hawks, an anthropologist from the University of Wisconsin-Madison, told MailOnline: 'We don't know of any physiological traits that make Neanderthals distinct, that is, traits that don't overlap. 'Almost every physical trait in Neanderthals overlaps in its variation with ours today, at least to some extent.' That means they wouldn't look like lumbering cavemen or women, but rather like a slightly different variation of humans. Denisovans, meanwhile, are a little more of a mystery. It was only this month that scientists identified the first Denisovan skull, and besides this, there are only small fragments of bone to go on. Based on the newly identified skull, experts believe that Denisovans would have had a wide face with heavy, flat cheeks, a wide mouth, and a large nose. These bones also show that Denisovans would have been exceptionally large and muscular people, much stronger than more slender Homo sapiens. Not all that different However, experts say that Homo sapiens, Denisovans, and Neanderthals might not have remained that distinct for long. These human species interbred widely during the periods they overlapped, and many modern humans carry at least some Neanderthal and Denisovan DNA. If these species hadn't vanished, they might have continued to interbreed and further intermix our genes. Dr Hugo Zeberg, an expert on gene flow from Neanderthals and Denisovans into modern humans from the Karolinska Institutet in Sweden, told MailOnline: 'In a way they never went extinct. We merged!' 'Probably the relatively low amount of Neanderthal and Denisovan DNA in present humans reflects the fact that modern humans [Homo sapiens] were more numerous. 'But with more chances of encounters, we might have more archaic DNA present in the gene pool of modern humans.' We're still learning about how ancient genes influence modern humans, so it's hard to say what effects this mixing might produce. Neanderthals and modern humans interbred when we overlapped and would likely continue to into the future. Scientists say that male Neanderthal and Homo sapiens female couples would have had the highest likelihood of producing fertile hybrid offspring (AI-generated impression) Could modern and ancient populations merge? Scientists say it is extremely likely that Homo sapiens, Neanderthals, and Denisovans would eventually merge if no species went extinct. The resulting species would have genetic traits from all three groups. Since modern humans reproduce faster and build larger communities, they would likely represent more of the genetic material. This is essentially what happened to these species in the past. Our modern genome contains traits passed on from breeding with these ancient species before they disappeared. But Dr Zeberg points out that Denisovan genes are responsible for 'high altitude adaptation for Tibetans and some influence on lip shape in Latin American populations.' Similarly, Neanderthal and Homo Sapiens hybrids would likely have a mixture of the traits of both species such as larger heads, longer limbs, and narrower hips. Over time, some scientists believe Denisovans, Neanderthals and Homo Sapiens might have merged into a single human species with a mixture of all the traits. Dr Bence Viola, a paleoanthropologist at the University of Toronto, told MailOnline: 'I think it would have been impossible for Denisovans and Neanderthals to retain sufficient genetic isolation to remain a separate population. 'We know that they interbred with modern humans whenever they came into contact, and so the more contact there is, the more mixing happens – so they would have become a part of us.' Would they fit in? We don't currently know very much about how Denisovans lived, but research now shows Neanderthals might have struggled to fit in with modern society. One of the leading theories for why Homo sapiens survived while other species dropped off is that modern humans essentially 'tamed' ourselves. Unlike modern humans, Neanderthals and Denisovans didn't evolve to be so sociable. Scientists say they would find it difficult to integrate into our hyper-social society (AI-generated impression) Could Neanderthals and modern humans coexist? Scientists aren't sure whether Homo sapiens and Neanderthals could have cohabited permanently. There is no evidence of violence between the two species and we know that groups met and interbred frequently. However, Neanderthals are not as pro-social as modern humans and lived in much smaller groups. Homo sapiens also have a long history of violence and discrimination targeted against other human groups. In large, connected modern cities this could lead to conflict between the two species. Modern humans developed genes that allowed us to become more sociable, develop larger social networks, and work with our fellow humans. Dr Noel says: 'Unlike their modern human contemporaries, Neanderthals lived in small, fairly isolated groups. 'If there was an accident that killed a number of their hunters or some other crisis occurred, they did not always have others to reach out to. As a result, their numbers would drop below what you need to be sustainable.' Dr Noel points out that research into Neanderthal genes suggests they were less cognitively flexible, had greater difficulties processing language, and lacked genes related to self-awareness, creativity, and behaviours intended to benefit others. 'In the highly connected world we all live in, I think Neanderthals would have been left behind, or at least, left out,' says Dr Noel. In a world where Neanderthals lived alongside other human species, this could really change the way society was structured. Professor Spikins says that while modern humans became 'tamer, more playful and more friendly to each other,' those changes came alongside 'being a bit easily led'. She adds: 'If Neanderthals were better at not "following the herd" and more of those tendencies were present, I bet much of our world would be different; they might not be easily swayed by social media!' How would the world be different? If Neanderthals and Denisovans hadn't gone extinct thousands of years ago, the world might be a very different place. From the evidence we have of these ancient species, we know that they lived in much smaller communities and had a far more limited impact on the land. In fact, Dr Zeberg points out that modern humans appear to be unique in the way that we modify the world around us through agriculture and large cities. One strange consequence of this is that a world where Homo sapiens are not dominant might mean a world without pets. There is no evidence that Neanderthals and Denisovans attempted to nurture relationships with animals through domestication - that means no horses, cats, dogs or even modern agricultural species like cattle and sheep. But with more of our relative anti-social genes, humanity may also have avoided some of its more destructive tendencies. Professor Spikins says: 'If Neanderthals had been the ones to survive, we might not have the problem we have with climate change, as their tendency to be more isolated within their separate groups might have limited how technology spread and got used, and how much the environment got exploited.' THE DENISOVANS EXPLAINED Who were they? The Denisovans are an extinct species of human that appear to have lived in Siberia and even down as far as southeast Asia. The individuals belonged to a genetically distinct group of humans that were distantly related to Neanderthals but even more distantly related to us. Although remains of these mysterious early humans have mostly been discovered at the Denisova Cave in the Altai Mountains in Siberia, DNA analysis has shown the ancient people were widespread across Asia. Scientists were able to analyse DNA from a tooth and from a finger bone excavated in the Denisova cave in southern Siberia. The discovery was described as 'nothing short of sensational.' In 2020, scientists reported Denisovan DNA in the Baishiya Karst Cave in Tibet. This discovery marked the first time Denisovan DNA had been recovered from a location that is outside Denisova Cave. How widespread were they? Researchers are now beginning to find out just how big a part they played in our history. DNA from these early humans has been found in the genomes of modern humans over a wide area of Asia, suggesting they once covered a vast range. They are thought to have been a sister species of the Neanderthals, who lived in western Asia and Europe at around the same time. The two species appear to have separated from a common ancestor around 200,000 years ago, while they split from the modern human Homo sapien lineage around 600,000 years ago. Last year researchers even claimed they could have been the first to reach Australia. Aboriginal people in Australia contain both Neanderthal DNA, as do most humans, and Denisovan DNA. This latter genetic trace is present in Aboriginal people at the present day in much greater quantities than any other people around the world. How advanced were they? Bone and ivory beads found in the Denisova Cave were discovered in the same sediment layers as the Denisovan fossils, leading to suggestions they had sophisticated tools and jewellery. Professor Chris Stringer, an anthropologist at the Natural History Museum in London, said: 'Layer 11 in the cave contained a Denisovan girl's fingerbone near the bottom but worked bone and ivory artefacts higher up, suggesting that the Denisovans could have made the kind of tools normally associated with modern humans. 'However, direct dating work by the Oxford Radiocarbon Unit reported at the ESHE meeting suggests the Denisovan fossil is more than 50,000 years old, while the oldest 'advanced' artefacts are about 45,000 years old, a date which matches the appearance of modern humans elsewhere in Siberia.' Did they breed with other species? Yes. Today, around 5 per cent of the DNA of some Australasians – particularly people from Papua New Guinea – is Denisovans. Now, researchers have found two distinct modern human genomes - one from Oceania and another from East Asia - both have distinct Denisovan ancestry. The genomes are also completely different, suggesting there were at least two separate waves of prehistoric intermingling between 200,000 and 50,000 years ago. But what they did not expect to find was individuals from East Asia carry a uniquely different type.
The Guardian
21 hours ago
- The Guardian
‘It's something that happens': are we doing enough to save Earth from a devastating asteroid strike?
It is a scenario beloved of Hollywood: a huge asteroid, several miles wide, is on a collision course with Earth. Scientists check and recheck their calculations but there is no mistake – civilisation is facing a cataclysmic end unless the space rock can be deflected. It may sound like science fiction, but it is a threat that is being taken seriously by scientists. Earlier this year, researchers estimated that asteroid YR4 2024 had a 3.1% chance of hitting Earth in 2032, before revising that likelihood down to 0.0017%. This week, new data suggested it was more likely to hit the moon, with a probability of 4.3%. If that happens, the 53- to 67-metre (174ft-220ft) asteroid previously called a 'city killer' will launch hundreds of tonnes of debris towards our planet, posing a risk to satellites, spacecraft and astronauts. Before that, in April 2029, 99942 Apophis – an asteroid larger than the Eiffel Tower – will be visible to the naked eye when it passes within 32,000km of Earth. This attention-grabbing close encounter has prompted the UN to designate 2029 as the international year of planetary defence. When it comes to apocalyptic asteroid strikes, there is precedent, of course. Most scientists believe such an event hastened the demise of non-avian dinosaurs 66m years ago. 'This is something that happens,' said Colin Snodgrass, a professor of planetary astronomy at the University of Edinburgh. 'Not very often, but it is something that happens. And it's something that we could potentially do something about.' As Chris Lintott, a professor of astrophysics at the University of Oxford, told the UK parliament's science, innovation and technology committee this week, the risk posed by an asteroid originating beyond our solar system is minimal. Instead, he said, the greater threat comes from those in our cosmic back yard. 'Most asteroids in the solar system exist in the asteroid belt, which is between Mars and Jupiter, but they become disrupted, usually by encounters with either of those planets, and they can move into orbits that cross the Earth,' said Lintott, who presents the long-running BBC astronomy series The Sky at Night. 'Then it's just a case of whether we're in the wrong place at the wrong time.' The chances of an enormous asteroid – the type that did for the dinosaurs – hitting Earth is admittedly low. 'We think there's one of these every 10m to 100m years, probably,' Lintott told the Guardian. 'So I think you'd be right to ignore that when you decide whether to get up on a Thursday morning or not.' Snodgrass said there were 'precisely four' asteroids big enough and close enough to Earth to be considered 'dino-killers', and added: 'We know where they are, and they're not coming anywhere near us.' But damage can also be done by smaller asteroids. According to Nasa, space rocks measuring about one to 20 metres across collided with Earth's atmosphere resulting in fireballs 556 times over 20 years. Many collisions have occurred over the oceans, but not all. 'Chelyabinsk is the best example,' Lintott said. In 2013, a house-sized space rock – thought to have been about 20 metres across – exploded in the air above the Russian city with a force of nearly 30 Hiroshima bombs, producing an airburst that caused significant damage and hundreds of injuries, mostly from broken glass. Less dramatically, in February 2021 a space rock thought to have been just tens of centimetres across broke up in Earth's atmosphere, with fragments landing in the Cotswold town of Winchcombe in the UK. Thankfully, the damage was confined to a splat mark on a driveway. The types of asteroids we should perhaps be most concerned about are those about 140 metres across. According to Nasa, asteroids around that size are thought to hit Earth about once every 20,000 years and have the potential to cause huge destruction and mass casualties. The space agency has a congressional mandate to detect and track near-Earth objects of this size and larger, and a suite of new technological advances are helping them do just that. On Monday, the first images from the Vera C Rubin observatory in Chile were released to the public. This telescope is expected to more than triple the number of known near-earth objects, from about 37,000 to 127,000, over a 10-year period. In just 10 hours of observations, it found seven previously unspotted asteroids that will pass close to the Earth – though none are expected to hit. Also in the offing, though not planned for launch before 2027, is Nasa's near-Earth object (Neo) surveyor. Armed with an array of infrared detectors, this is 'the first space telescope specifically designed to detect asteroids and comets that may be potential hazards to Earth', the agency says. Lintott said: 'Between those two, we should find everything down to about 140 metres.' He said such observations should give scientists up to 10 years' warning of a potential collision. The European Space Agency (Esa) is planning a near-Earth object mission in the infrared (Neomir) satellite. Slated for launch in the early 2030s, this will help detect asteroids heading towards Earth that are at least 20 metres in diameter and obscured by the sun. Assessing the emerging capabilities, Edward Baker, the planetary defence lead at the UK's National Space Operations Centre (NSpOC) at RAF High Wycombe, said: 'I think we're in a good place. I can't see a situation like [the film] Don't Look Up materialising at all – though I wouldn't mind being portrayed by Leonardo DiCaprio.' As our ability to spot near-Earth asteroids increases, Lintott said, we should get used to hearing about asteroids like YR4 2024, which initially seem more likely to hit Earth before the risk rapidly falls towards zero. He described the shifting probabilities as similar to when a footballer takes a free kick. 'The moment they kick it, [it looks like] it could go anywhere,' he said. 'And then as it moves, you get more information. So you're like: 'Oh, it might go in the goal,' and then it inevitably becomes really clear that it's going to miss.' Of course, scientists aren't just monitoring the risks to Earth. They are also making plans to protect it. In 2022, Nasa crashed a spacecraft into a small, harmless asteroid called Dimorphos that orbits a larger rock called Didymos to test whether it would be possible to shift its path. The Dart mission was a success, reducing Dimorphos's 12-hour orbit around Didymos by 32 minutes. In 2024, Esa launched a follow-up to Nasa's Dart mission, called Hera. This will reach Dimorphos in 2026 and carry out a close-up 'crash site investigation'. It will survey the Dart impact crater, probe how effectively momentum was transferred in the collision and record a host of other measurements. Esa hopes this will provide crucial insights that can be used to make deliberate Dart-style impacts a reliable technique for safeguarding Earth. 'Dart was much more effective than anyone expected it to be,' Lintott said. 'And presumably that's something to do with the structure of the asteroid. I think we need to know whether Dart just got lucky with its target, or whether all near-Earth asteroids are like this.' For the most part, scientists say the threat of an asteroid strike does not keep them up at night. 'We're safer than we've ever been and we're about to get a lot safer, because the more of these things we find, the more we can spot them on the way in,' Lintott said. As Esa has quipped on its merchandise: 'Dinosaurs didn't have a space agency.'
Telegraph
2 days ago
- Telegraph
Why cats prefer sleeping on their left side
Cats prefer to sleep on their left side to protect themselves from predators, a study has found. The pets sleep for up to 16 hours a day and often curl up or stretch out for a snooze in opportune places. But the way the animal settles down is not random, and there is an evolutionarily hard-wired logic underpinning it, according to a study from the Ruhr University Bochum in Germany. Scientists found cats lie on their left side around two-thirds of the time, which shows that it was done deliberately. They looked at clips on YouTube of more than 400 sleeping cats and logged which side they were sleeping on. Data revealed that 266 of the cats (66.5 per cent) were on their left side, leaving scientists to conclude this was a survival trait from their history in the wild. Sleeping on their left side means when they wake, their left eye is able to see the local area unobstructed by the cat's own body. This visual information is then processed by the right side of the brain. This hemisphere is what processes threats and is responsible for escaping danger as well as knowing an individual animal's position. This puts the cat at an advantage compared to if it was to sleep on its right side – when the information is processed by the left side of the brain, which is less specialised to aid a swift escape. Anti-predator vigilance This leftward preference is just one of the many ways in which cats protect themselves. 'Sleep is one of the most vulnerable states for an animal, as anti-predator vigilance is drastically reduced, especially in deep sleeping phases,' according to the study. 'Domestic cats are both predators and prey (e.g. for coyotes) and sleep an average of 12–16 hours a day. 'Therefore, they spend almost 60-65% of their lifetime in a highly vulnerable state. To reduce predation risks, cats prefer to rest in elevated positions so that predators are more visible to them and the cats, in turn, are more visually concealed from predators. 'In such a spot, predators can access cats only from below. Thus, their preference for resting in an elevated position can provide comfort, safety, and a clear vantage point for monitoring their environments. 'We hypothesised that a lateralised sleeping position further increases the chances of quickly detecting predators (or to identify careless prey) when awoken.' Threat-processing leftward bias Pregnant cows are known to prefer their left side while sleeping for a similar reason, experts believe. The scientists also found that the pawedness of a cat, whether it preferred its left or right side, is likely not to blame for the sleeping preference. A 2017 study found that male cats tend to prefer their left paws and females are more right-paw dominant. 'We are inclined to believe that the significant leftward bias in sleeping position in cats may have been evolutionarily driven by hemispheric asymmetries of threat processing,' the scientists add in their paper, published in the journal Current Biology.
