Article curated by Ginny Smith
Since Charles Darwin published On the Origin of Species in 1859, the theory of evolution by natural selection has been able to explain how the diverse and often strange-seeming species that cover our planet came into existence. But there are some areas of evolution that scientists are still working to understand. From specific traits found in animals and humans to the very origins of life on earth, there are still plenty of things we don't know about evolution.
Over the past 4 billion years, life on earth has expanded from simple single celled organisms to the diversity of plants and animals we see today. Although multicellular organisms have only been around for a million years, they have evolved a myriad of different survival tactics, each perfectly adapted for their particular environment. But how some of these adaptations came about is still a mystery to scientists.
Although animals in some form or another exist almost everywhere on the planet, there are some areas where the diversity of species is much higher than in others. The tropics, for example, seem to harbour many more species than higher latitudes. However, it is difficult to even begin studying this problem when we are yet to get a handle on exactly how many different species of plants, animals and microbes exist on earth. It is a tough challenge, as the variation within a species can often appear to be as great as the differences between two closely related, but separate, species.
Speciation, the evolution of one species into two, can occur for many reasons, including environmental change or sexual selection. Anything that separates a population and prevents them interbreeding will encourage speciation, but these factors are extremely varied and difficult to pin down. However understanding this process may help us to protect the diversity of our ecosystem, and prevent further species from going extinct.
Origins of Life
In order to fully understand how life evolved on earth, we need to know where it originated. Popular theories suggest that hot springs and hydrothermal vents are the most likely candidates, as they contain the right kinds of chemicals, and the energy needed to spark life. However there isn’t enough evidence to say definitively that this is where life began. 
Using DNA sequencing, researchers are trying to put together a tree of life, in order to trace our ancestors back to their origins. Currently, life has been traced back to an aquatic microorganism that lived in high temperatures, providing support for the theory that life evolved in hydrothermal vents. However there are still plenty of unanswered questions that mean this idea isn't yet agreed upon by science.
One competing theory is that the building blocks of life actually came from outer space. Glycine is the smallest of the 20 amino acids that most commonly make up proteins, the large, complex molecules that do most of the work in our body. Some people think that the first glycine molecules arrived when a comet carrying them collided with Earth. However, little conclusive evidence has been found as it is very hard to distinguish actual interstellar glycine collected by probes from glycine that contaminated the probe before it left Earth. Another way of confirming the presence of interstellar glycine is by the analysis of spectral data collected by telescopes, but reported instances of any kind of building block of life, such as amino acids or RNA, are always widely disputed. The discovery of any molecule that could explain where we came from in space would be hugely important, as it could also imply that life in the universe could be common and exist in places other than Earth.
Delve deeper into origin of life.
One of the earliest types of life to evolve on earth was viruses, but their origins are unknown. Viruses are like no other form of life on earth, in that they cannot replicate on their own, rather hijacking the replication mechanism of their host cell after infecting it. One theory is that they were originally small parasitic cells, which then lost their replication mechanism. However there are many differences between viruses and small cellular parasites- these would need to be explained before this theory could be fully accepted.
Viruses consist of a strand of DNA inside a protein coat, so another theory suggests that they formed from parts of a DNA molecule which escaped from a cell. However viruses also contain complex structures to help them attack and hijack a host cell, and it is not clear how they could have survived before evolving these. Others think that they evolved around the same time as the first cell, from a molecule of protein and nucleic acid, but as viruses cannot survive without a host, it isn’t easy to see how they could have evolve separately.
Due to their tiny size, and lack of any bony parts, viruses do not fossilise, so it is difficult to study their past. However discoveries of new species, such as a virus much larger than was thought possible, found during ocean exploration, will lead to a better understanding of how they live. Hopefully, this will also shed some light on how they evolved.
For life as we know to have evolved, an oxygen rich atmosphere needed to develop. Around 2 billion years ago, the abundance of oxygen in our atmosphere suddenly increased dramatically, and with it did the variety of life on earth- with the first multi-celled organisms appearing. However why this change came about isn't known.
It is thought that single-celled cyanobacteria started to photosynthesise in the pools of water that existed on Earth's surface, producing oxygen. Initially, this oxygen reacted with iron and other chemicals dissolved in the water or present in rocks, but once these 'oxygen sinks' were full, it started to build up in the atmosphere. However the speed of the build up, and the levels reached, are hard to explain.
One ideas is that the presence of free oxygen allowed the build up of ozone, which protected the bacteria from harmful UV rays, in turn allowing them to produce more oxygen. Another theory suggests that the composition of other gasses in the atmosphere changed, allowing oxygen to build up. Free oxygen will react with hydrogen produced by volcanoes to form water, taking it out of the atmosphere. If the levels of hydrogen went down, so the oxygen could increase. But how could the hydrogen be removed? One idea is that the oxygen produced by bacteria rapidly reacted forming oxides and was buried, leaving the hydrogen with nothing to react with, so it disappeared into space. When the oxygen was later released from it's burial site, there was little hydrogen for it to react with, so levels increased.
Once it had evolved, for life to establish itself on earth, it needed a way to pass on it's genetic material to the next generation. Initially, this is likely to have been through asexual reproduction (like that still seen today in bacteria or yeast). This is quick & easy, but produces identical offspring meaning bad mutations can build up. Another problem comes if the environment changes as its unlikely any of the offspring will have adapted so a whole line can be wiped out. To get around this, you need to mix your genes with someone else's- there are other ways of doing this, but sex is one of the best.
The first sexual encounters were probably between two organisms that were effectively the same- not male or female, but both somewhere in between, with similar sized gametes- some fungi and algae still mate this way. Over time, in many species these evolved into two mating types, A and B- we aren’t quite sure why this happened, although it may be to do with keeping our mitochondria in check by only one of the two parents passing them down. Once this had happened, it was advantageous for each of these mating types to specialise- one to make many, small gametes so they could have as many offspring as possible and the other to invest more time and resources into ensuring the health of the offspring they produce- and so male & female came into existence.
This, in time, led to the ‘choosy female, ardent male’ system we see in modern animals and interestingly there is evidence that this is an advantage to the species as it improves the fitness of the next generation.
Even if we managed to solve the mystery of the origins of life, there are plenty of stepping stones en-route to the animals we see today we are yet to understand. Many animals, including mammals, are supported by an internal system of bones, known as an endoskeleton. Some other animals, such as insects and crabs, have their skeletons on the outside. It is not clear whether one type of skeleton developed first, and the other evolved from it, or whether they evolved separately from an ancestor with no skeleton. The likelihood is that skeletons originally developed as soft tissue, and only mineralised into the hard structures we see now over time. As soft tissue does not tend to fossilise well, this means we have no records of the first animals to have these structures. 
We know that multi-cellular life originated in the oceans, so one of the biggest mysteries in its development is how early animals first came out of the sea onto land. There are a huge number of adaptations needed to make this possible- one of them being legs!
The first tetrapods - four legged land living creatures - evolved from fish, with their forelimbs developing from the pectoral fins of the ancestral fish. These fins contain three or more bones connected to the fish's 'shoulder', where-as tetrapods only have one. How this change happened isn't well understood, but a recent breakthrough may hold the beginnings of an explanation. Genetic research into catsharks, which are similar to the kinds of fish tetropods are likely to have evolved from, has shown there are differences in their levels of certain genes expressed in the posterior (back) side of their fins compared with the corresponding place on a mouse's leg. These genes are involved in telling regions of the developing limb where they are located, so producing a balanced limb with a front and a back side. The research suggests that a change in the expression of these genes could have been vital in the change from fins to limbs- as the fin became 'posteriorised' and the anterior bones were lost, it began to look much more like the tetrapod limbs we see today.
As well as having a backbone, mammals are defined by their ability to produce milk to feed their offspring. Although some other classes of animals feed their young via secretions, mammals are unique in having a mammary gland which produces the milk. Again, the problem in determining the evolution of the structure lies in it being made of soft tissue, so not fossilising well. Although it is clear that lactation confers an evolutionary advantage, the lack of fossil records means we cannot say for certain when, and via what intermediary stages, the mammary gland evolved.
When exactly the first mammals evolved is another subject that is under debate. It used to be thought that they first appeared less than 200 million years ago, however new fossils of a prehistoric squirrel-like creature discovered recently in China have potentially pushed this back to at least 208 million. These are the first fossils of these creature to be found that can confirm for sure that they were mammals- previously all that had been found were teeth and broken jaws. Although these fossils were only about 160 million years old, the researchers argue they add to the body of evidence suggesting mammals arose millions of years earlier than was previously thought.
Many mammals, as well as other animals, have evolved to live in social groups. But how these groups are formed and how the size of the group is decided isn't clear. Models can predict group size using ecological factors, such as the balance between availability of food (larger groups require more food) against the benefit in protection from predators, but it is much harder to work out the social factors that affect the size of the groups. This is often studied in primates as their groups are very large and variable, and can split and then reform when necessary . Social factors can include male-male competition and the risk to infants brought by larger group with more males. Factors like these can make the number of females in a group especially difficult to predict, and the number of different factors involved makes it difficult to develop a model that can make accurate predictions. 
A behaviour which is common to all but the simplest of animals is sleep. Humans spend roughly a third of our lives doing it, yet scientists still don’t really know what sleep is for, or why we need it. However, we do know that going without sleep will cause death sooner than going without food . This suggests that it must have some incredibly important and evolutionarily ancient function.
Some theories suggest that sleep is important to keep the brain in working order. It may help with the organisation of memories, or allow the synapses (nerve junctions) in the brain to be cleared of toxins that build up during the day. Alternatively, it may allow restoration of the body, or simply keep the organism away from potentially hazardous situations for a length of time. However, none of these factors have been shown to be powerful enough to be a primary reason for sleep to have evolved, leaving its main function still unknown.
Delve deeper into Why do we sleep? [SCIENCE VIDEO].
Over the history of life on earth, there have been five mass extinctions. While we have some information about when they occurred, their causes, and the extent of the extinctions, we are still lacking some important details. For example, we are yet to uncover the cause of the Late Devonian extinction, or how many phases the extinction at the end of the Ordovician Period happened over. We know there was an asteroid impact around the time of the end-Cretaceous extinction, but whether it was the sole cause, or simply a contributing factor is yet to be discovered. It would also be interesting to know how long it takes for species to recover after these events. Some people argue that we are in the midst of a 6th, human driven, mass extinction so if we are to prevent further destruction of biodiversity it is important to understand the drivers for, and the consequences of, past mass extinction events.
Some of the most fondly remembered extinct animals are woolly mammoths, which lived on arctic islands as late as 1700BC. It is still not known what it was that finally killed off these furry giants- it could have been human hunting, although there is little evidence of it left behind. A virus might have led to their demise, or it could have been a change in the habitat, or a large weather event. Alternatively, it could be that the island they lived on just couldn’t support them, so as the ice bridge to the mainland melted they became stranded and could no longer survive. It remains to be discovered which of these theories best fits with the pattern of extinction seen.
Delve deeper into extinction.
There are other things we don't understand about these incredible creatures as well. For example, they were smaller than their ancestors- about the same size as modern African elephants. There must be some reason this smaller size was advantageous to them, but we haven’t yet determined what it was.
We also don't know why Neanderthals went extinct, roughly 30,000 years ago. These ancient hominids were developing in Europe and Asia while modern humans were evolving in Africa, and there is an increasing amount of evidence suggesting our ancestors crossed over with Neanderthals when they reached Europe. Uncertainty in methods used to date bones and artefacts, however, has made it difficult to be precise about the extent of this crossover.
One theory for their demise is that our more intelligent ancestors simply out-competed the Neanderthals, while other people believe that changes in the climate contributed. Recent DNA studies, however, hint at a more intriguing possibility. Modern people in Asia, Europe and New Guinea have been found to have 2.5% Neanderthal DNA, suggesting that our ancestors interbred with Neanderthals when they met. This could mean that rather than becoming extinct in the traditional sense, Neanderthals could have simply been assimilated into our modern human lineage- and live on in many of us .
Specific animal adaptations
When exposed to certain wavelengths of UV light, scorpions glow a bluish-green colour. While a nice party trick, it is not clear why this ability would have evolved. The glow is due to a material in the scorpion’s exo-skeleton, which absorbs UV light, and emits photons in the visible range. As the animals grow, each replacement shell contains more of this material, meaning bigger and older scorpions glow brighter than young ones. Some people believe that this ability may work as an extra eye, allowing scorpions to detect even tiny amounts of star light, and so seek shelter in the shadows. Other theories suggest it may act as a sunscreen, protecting the mainly nocturnal animals if they have to go out in the daytime.
Scorpians aren't the only creature to glow- some deep sea bacteria actually produce their own light, a form of bioluminescence. These bacteria often form symbiotic relationships with squid or angler fish; the bacteria get nutrients from the animal, and the animal uses their glow to attract prey or repel predators- there is a simple evolutionary benefit for each of them to work with the other.
However, we know that these bacteria evolved before the animals they partner up with even existed, so why did they evolve & maintain bioluminescence? One suggestion was that they used the light to signal to each other; the fact that they only glow when there is a big enough group of them seems to support this hypothesis. However, we now know this isn't the case- they actually use pheromones to signal instead. In fact, these bacteria don't have any light sensors- so they have no way to detect each others' glow!
Another idea is that the light could be a by product of another, evolutionarily beneficial process. Producing the light uses up free radicals (reactive chemical species that can damage cells)- it could be that this is the primary use, and the light is simply a waste product of this process. Or, it could be that they produce the light because they need it (despite not being able to see it!) . The DNA repair enzyme photolyase requires visible light to work-it could be that a bacterium that produced light was able to repair itself a bit better in the darkness of the deep ocean, and so the trait got passed along. Which of these suggestions, if any, is the real answer is an active area of research.
Another animal adaptation that has not been fully explained is the long necks seen on giraffes. Most people believe that giraffes have long necks to allow them to reach leaves high up on trees which other animals could never reach. However, some modern giraffes don’t seem to eat leaves high up in trees very often, even when food is scarce, meaning this explanation isn't likely to be the whole story.
Male giraffes use their long necks to fight over females, and the giraffe with the longer neck usually wins. Females also tend to prefer the male with the longer neck. This ‘sexual selection’, the evolution of traits that serve no real purpose except to make the male more attractive to the female, is common throughout the animal kingdom- peacocks are an obvious example. But in these cases, it is usually only the male that evolves the trait- there is no reason for the female to. Long necks are costly, as they require a powerful heart to pump blood to the brain. So the fact that females also have long necks suggests that sexual selection alone can’t explain this trait.
Despite being one of the best cited examples of the difference between Lamarkian and Darwinian ideas of inheritance, there is no scientific consensus on the main driving force for the lengthening of giraffes’ necks. More evidence from fossil records and studies of modern giraffes will hopefully help us to understand exactly how and why their necks evolved.
Understanding how animals evolved in the past to fit their ecological niches is challenging, but equally difficult is predicting how the changing world will affect the survival of species in the future.
One big challenge for animals is the rapidly changing climate on earth. Scientists have found that some small sea creatures evolve an increase in heat tolerance of only half a degree over 10 generations. This is surprising, as these animals can tolerate fluctuations of 20 degrees in the wild, but it suggests they may already be living at the edge of their heat tolerance . Unfortunately, if this is the case, they are unlikely to survive if climate change continues at its current rate.
However, studies on other animals are more promising. A team have been studying a population of Great Tits near Oxford UK for the past 50 years. They have found that these and other small birds could cope with an increase of half a degree Celsius per year. The main predictive factor for the survival of these birds is the caterpillar population, so the birds now lay their eggs 2 weeks earlier than they did 50 years ago, to capitalise on the boom in caterpillar numbers in spring . This ability to adapt to changing conditions suggests they can likely survive the current climate change predictions.
Delve deeper into climate change.
As well as learning about animals in order to better protect them, we humans can also learn from animals. Often evolution has come up with solutions to problems which are better than those we could have developed ourselves, so by learning from nature we can increase our productivity.
One example of a remarkably efficient system developed via evolution is seen in ant colonies. Ants have managed to inhabit the majority of environments on Earth, from the desert to the countryside to the tropical rainforest, by working together. No single ant ever sees the big picture- each does its own job, learning from those it interacts with. This allows the colony as a whole to change and optimise its system effectively, an ability we would love our computers to have. By learning more about how the networks within ant colonies work, we may be able to apply the ideas to our own engineering, to build better more efficient systems. 
Humans, like other animals, have evolved over millions of years. We are intelligent, social, flexible animals, and our bodies and brains have evolved to help us survive in countless different environments. However, there are various physical characteristics and behavioural traits that are found in humans that don’t seem to be helpful to us. It is unclear why we have these traits- it may be that they provided an advantage to our ancestors that is no longer obvious, or it may be that they are simply a by-product of some other trait that did help us to survive. By studying fossil records, and the items left behind by our ancestors, scientists hope to shed light on the evolutionary value of these traits.
Probably the thing that most distinguishes humans from other animals is our intelligence, and our enormous brain size, in relation to the size of our bodies. This produces many disadvantages for us as a species, including contributing to difficult childbirth, as the baby’s head is so large when it is born. It also means infants are born at an early stage of development, so are very helpless for many months after birth. Our brains take such a long time to mature that infants stay with their parents for around 18 years- a huge burden on the parents. So why do we have such big brains, if they cause us so many problems?
One theory is that brain size relates to complexity of social group structure. Animals that live in groups need to remember each individual, keep track of who is dominant, who is an ally, and who won’t play fair or return a favour. This is complicated, and requires a big brain, which in turn can be used for other beneficial traits, such as using tools, and adapting to different environments. However, this does not explain why the human brain is so much more complicated than that of other social animals .
It has also been suggested that it is not brain size per se, but the size of the neocortex, the evolutionarily newest top layer of the brain, which correlates with intelligence. Humans have a neocortex which is folded, allowing a bigger volume to fit into our skulls, meaning we can cram even more processing power into our already oversize heads, without expanding them further.
One thing our big brain provide us with is a remarkably powerful response to music- it can trigger emotions and memories, and cause us great joy or sorrow. It can even induce physical sensations, such as foot tapping or goose bumps. And yet two people may have completely different responses to the same piece of music. Why this is isn’t fully understood.
Music that we enjoy causes activation in a region of the brain called the nucleus accumbens, which tells us that something, usually food or sex, is rewarding . This suggests that it may have had some kind of importance for our early ancestors, other than simply enjoyment. Some experts argue that it developed alongside language, or before it, as a means of communication. Others think it was important to bond groups, or involved in sexual selection. Still others suggest it is simply a by-product of some other, more advantageous trait. How and why it evolved may remain a mystery, but as a species, our love for music is something that can't be denied.
The culture that you grow up in, and the music you are surrounded by plays a large role in the kind of music you choose to listen to. However, there are limits to this (many of us have experienced our parents telling us to “turn that racket off!” when we are listening to the latest hits). So what is it that means that one person loves thrash metal, while another can’t stand it and would rather listen to experimental jazz? It is not clear whether there is a genetic role in musical preference, or whether it is solely down to the influence of culture. Scientists are also unsure why specific songs appeal to different people, but the reactions we have to music may be able to give us some insight into the workings of our brains. Music is used in various situations, ranging from setting the scene in a film to helping with rehabilitation, and more information about how we react to different types of music can only help it to be used more effectively.
Despite our large brains, and seeming intelligence, we humans are a superstitious bunch. This is due to our predisposition to look for patterns- everywhere we go we are searching for causes and effects. Most of the time, this serves us well- if one day whilst clambering over wet rocks we slip and hurt ourselves, we will learn that water causes rocks to be slippery, and will be more careful next time. We have used our experiences to determine causality, and used that to learn something that is helpful for our survival. However, sometimes we ‘see’ these connections where they aren’t actually present. Many people have a lucky mascot- this tends to be something the person happened to be wearing or carrying when something good happened, and so the good event was attributed to the object. Often we even know, logically, that a pair of underpants isn’t going to make any difference to how we do in our exams, but we still wear them, ‘just in case’.
A possible extension of our superstitiousness is religion- but why are we the only animal to have evolved it? Anthropologist and author Barbara King argues that while non-human primates are not religious, they do show some traits that would have been necessary for the evolution of religion; high intelligence, a capacity for symbolic communication, a sense of social norms, realization of "self"and a concept of continuity. Elephants actually demonstrate rituals around their deceased, which include long periods of silence and mourning at the point of death and a process of returning to grave sites and caressing the remains. However it is clear that no other animal has developed organised religion in the way we have.
There are two ways religion could have developed-either it evolved due to natural selection because it provides an advantage, or it is an evolutionary by-product of other mental adaptations. Religion is expensive- it requires time and sacrifice, and it is also counter-intuitive, so there must have been a real benefit linked to it for it to have evolved.
One theory is that religion is an adaptation for cooperation- making groups more cohesive. By including ever-watchful ancestors, spirits and gods in the social realm, humans may have discovered an effective strategy for restraining selfishness and building more cooperative groups. Alternatively, it could be a by-product of evolved, non-religious, cognitive functions, such as a brain that is large enough to formulate complex ideas, an understanding of causality necessary for tool use, or our ability to link cause and effect and detect agents who are responsible for certain events.
Interestingly, stimulating the temporal lobes makes many people sense a ‘presence’ and temporal lobe epilepsy gives sufferers religious experiences, suggesting there is a ‘religious area’ in the brain. But how or why this exists is still unknown.
Alongside music and religion, there are other traits that seem to be common across human cultures. One of these is laughter, which can be prompted by a variety of situations, but is often involuntary. It is clear that laughing with another person is a powerful bonding tool, and there is evidence that we laugh more when with other people. However, we don’t yet understand why laughter occurs, and there is more to be learnt about the powerful effect it can have on the brain.
Another involuntary and often irritating behaviour is blushing- a reflex that occurs in some humans when they feel embarrassed. The feeling of embarrassment causes the release of the hormone adrenaline, dilating the blood vessels in your face and increasing the blood flow, causing heat and redness.
Humans are the only animal known to blush, but it seems to be common amongst humans of every race. It is, however, difficult to determine what the evolutionary advantage of a physical display of embarrassment would be. Humans are a highly social species, and use many different forms of communication, but it is likely that the earliest forms involved body language and facial expressions. It may be that blushing was an early way to signal an apology, showing that you regretted an action, and you respected the person you were communicating with. This may have avoided punishment-whether physical violence, or simply being shunned by the community, which would have had devastating effects on an early human’s survival chances. Or it could just be a side-product of another bodily process which, while not providing any advantage, also didn’t disadvantage the blusher, so was not selected against.
If laughter and blushing are difficult to explain, a more wide-reaching question is why we have emotions at all. Darwin argued that emotions evolved as a pre-verbal way of communicating. Facial expressions can demonstrate you are in need of help, sorry for something you have done, or even warn others to stay away if you are angry. However some people think there is a simpler explanation- the expressions evolved for one reason, and were then secondarily useful to tell what other people were feeling. For example, the widened eyes of fear could just be to help us to see better, and the disgust expression a way rejecting poisonous food.
Delve deeper into emotions.
Emotions are seen in humans throughout their lives, from infancy to old age. But there are some intriguing abilities that young infants have which are lost with age, such as the Palmar Grasp Reflex. Most people have experienced meeting a young baby, only to have them hold on to your finger with a vice-like grip. But why do babies who are unable to do almost anything else have such a strong grip reflex? It is even said that a baby could hang from a washing-line for up to two minutes- a feat that a lot of adults would struggle with!
Some people believe that this vestigial reflex dates from when we were hairy apes. Infant apes and monkeys are often carried on their mother’s back, and have to cling on to their fur to prevent themselves slipping off as she swings through the trees, so the grip reflex could be a remnant from when we carried our babies the same way. Others, however, argue that the reflex still serves a function, although what this function is remains a mystery.
Our childhoods are dominated by play, and it is thought to be a vital way for children to learn about the physical and social world around them. However we don't know which aspects of it have this role. Play is common across the animal kingdom, but only in animals with the highest intelligence and most complex lives, such as apes and dolphins. This suggests it has an evolutionary advantage for those animals, but not for simpler animals like fruit flies. What exactly this advantage is, however, is not known, and it could equally be that it is only more intelligent animals that have the time, or the extended childhoods, to allow play to occur.
As we grow up, our first adult teeth begin to push out our baby teeth. Many of us, however, don’t have our final adult teeth, the wisdom teeth, come through until we are in our twenties. This process is, at best, irritating, and at worst can cause real problems, with some people having to have the teeth removed as they will not fit in their jaws. Why we have these teeth at all is a question of debate.
Some people believe that when our diet consisted of courser food, they were important, but since our diet changed they have become redundant. However, there is little evidence to support this idea. Throughout the course of human evolution, the jaw has shrunk in size, so it may just be that while evolution has shrunk our mouth, the wisdom teeth did not cause our ancestors enough bother for there to be a selection pressure to lose them, so we are stuck with too many teeth for our mouths.
For many people, a part of growing up is being told for the first time that you need glasses. More and more people in the developed world need them to treat short-sightedness or myopia. The increasing prevalence is far faster than the process of evolution, so the cause is likely to be environmental. Spending a lot of time lookng at close-up objects, like TVs and books has been blamed, but research has failed to find a strong correlation. However, in countries such as China where children spend more time studying, the rates are higher. Recently, work has suggested that the important factor is how long a child spends outside, not how long they spend staring at something close. The mechanism behind this (and whether this is related to the distant view or natural light itself) is as yet unknown. Investigators are still exploring the extent of this influence and, to combat the huge problem of enforcing outside play, looking into questions such as whether using sunlight-mimicking SAD lamps would be just as effective.2
Another human mystery is the existence of homosexuality. Sexual activity between individuals of the same sex is common in animals, but very few show an exclusive preference for the same sex- for obvious evolutionary reasons, it would seem a bad strategy! So why does it exist in humans?
There are hints that there might be a genetic component- two genetic regions have been tied to male homosexuality. However relevant individual genes remain to be found. It is well accepted that there is a heritable component to homosexuality, but it can't explain everything, and the interaction between genetics and environment is hugely complex.
In fact, even your pre-natal environment may be important. Each additional older brother increases a man's chance of being gay by a third- going from a 3% chance with the first son to a 6% chance for the fourth. This only holds true for biological older brothers, and could because of the effects of the mother's immune system on the developing brain or changes in hormone levels. Levels of certain brain chemicals may also be involved- wiping out serotonin in mice changes their preference from the opposite sex to the same sex. At the moment, however, we don’t know whether this finding can be transferred to the infinitely more complicated sexual preferences of humans.
One suggestion for why homosexuality could have evolved to be so prevalent is the idea that the same gene that makes men homosexual might make their female relatives more fertile, meaning it would have been selected for. However, this isn’t enough to explain the complexities of human sexuality. In fact, it could only explain about 20% of the cases studied, so further research is needed to fully explain the prevalence of human homosexuality.
This article was written by the Things We Don’t Know editorial team, with contributions from Ed Trollope, Jon Cheyne, Freya Leask, Ginny Smith, Cait Percy, Johanna Blee, Rowena Fletcher-Wood, and Joshua Fleming.
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