The Land Massive

Life is hard for wild animals, being fraught with the perils of predation, starvation and sickness. Only the strongest manage to survive into adulthood and breed, which means that only the best genes are passed into the next generation. This is the major driving force behind evolution and is responsible for the staggering diversity of life on our little planet.

One of the most obvious adaptations that some species have is their tendency to grow to enormous sizes. This is typically a defense against predators and takes advantage of the 'ten times larger' rule. Simply put, this rule states that a terrestrial animal becomes safe from predation if it can grow ten times heavier than their largest predator, since their predators are unable to develop jaws strong enough to kill them.

Obviously, this rule is not absolute. It doesn't take mankind in account, as our technology and sentience allowed us to overcome it as cavemen, nor does it take into account predators growing larger themselves. If all of the prey species in an area begin to grow larger, this will itself drive a size increase in predators. Bigger prey means more meat per kill, which is able to fuel more muscle mass and only the largest predators will be able to make kills (so only the genes for a bigger body size will be passed down into subsequent generations).

This means that predators and prey are locked in a constant arms race where each is trying to become bigger. Over time, the size of each species will grow bigger until size becomes limited by another factor - such as the availability of food - and is therefore no longer advantageous (where upon it's selected against and a 'maximum' size of the species has been reached).

Animals have reached unbelievable sizes in the past, both in the oceans and on land. Dinosaurs are the obvious example of megafauna, with some species becoming so heavy that the ground used to tremble as they walked! Sadly, there aren't animals this impressive anymore (certainly not on land, anyway), but we do have some truly huge beasts roaming our continents. Here's a list of the top ten largest animals that are alive today (sorted by average weight):

10: Wild (Asian) water buffalo: 770kg/3.5m long (max weight: 1, 250kg)

Also called the Asian or Asiatic buffalo, the wild water buffalo, Bubalus arnee,  is endemic to Southeast Asia. As well as its obvious bulk, the bovid is famed for having the widest horn-span in its family, which can be as wide as 2m from tip-to-tip in large males.

Wild water buffaloes are gregarious and live in tight, matriarchal groups as a defense against predators. Each group is called a 'clan' and is made up from related females and their young offspring. A clan can be as large as 30 individuals and often band together with other clans to form large herds (for further defense) that can consist of more than 500 individuals!

To avoid inbreeding, young males actually leave their clan once they become sexually mature and form small 'bachelor' groups. These groups consist of about 10 males and generally spend the dry season away from females. In breeding season, the young males rejoin a herd where they are polygynous and mate with multiple females.

9: Black rhino: 1, 150kg/3.5m long (max weight: 1, 900kg)

Black rhinos have two horns on their snout, which are made from keratin - the same protein as hair and nails. Their horns can grow as much as 3 inches a year and individuals have been found with horns that are longer than five feet! Sadly, their horns have been their downfall as it is coveted for Asian herbal medicines and ornaments in the Middle East, meaning that the black rhino is now classified as 'Endangered' by the IUCN.

There are four species of black rhinos (of the genus Diceros), which are actually grey. Their skin looks darker because they frequently wallow in mud to help keep cool through the hottest parts of the African day. Their size and rotund shape means that they actually retain body heat, so black rhinos are usually inactive during the hottest hours and rest beneath shade if the cannot find shallow water. During gloaming, when it's cooler, the rhinos feed by browsing shrubs and the lower branches of trees.

8: Walrus: 1, 200kg/3.4m long (max weight: 2, 150kg)

Odobenus rosmarus - the Latin taxonomic name for the walrus - literally means "tooth-walking sea-horse". This name stems from the tendency of walrus' to drag themselves onto pack ice using their tusks for purchase. It should be noted, however, that walrus tusks are actually elongated canines. These teeth are so large that they can grow as long as 1 meter in big males!

Walrus' are found throughout the Northern Hemisphere in the Arctic Circle and some subarctic regions. Walrus' once covered most of the frigid north, but their numbers were decimated by hunting in the 19th Century and their populations are now discontinuous; being limited to certain regions. That being said, they can still be found through most areas of shallow water, where they like to dive and crawl across the bottom of the sea bed to feed off crustaceans, molluscs, amphipods and slow-moving fish.

7: Giraffe: 1, 400kg/6m tall (max weight: 2, 150kg)

There are nine distinct species of giraffe, which are classified under the genus Giraffa. Growing to heights of 5 - 6m, giraffes are the tallest living animal and have unique patterns of spots in their fur. Their spotting is very similar to human fingerprints and can used to identify an individual. Interestingly, their spots also reveal the relative age of a giraffe as they grow darker as throughout their lives.

Giraffes are herbivores and use their long necks to reach leaves that are high up in the canopies of trees. This allows them to access food that many animals are unable to get at, thus allowing them to exploit a very specific niche. Despite this being an obvious use of their long neck, many scientists are skeptical that this is why it first evolved. Their neck is simply too exaggerated for access to slightly more food to have driven its appearance and many scientists now believe that the length of their neck was initially a sexually selected trait.

During mating season, male giraffes fight for the right to mate with females. They have an interesting way of dueling called 'clubbing', where they literally use their heads as bludgeons to strike their opponent with. As explained by the mechanisms of physics, a giraffe's head will hit harder the longer its neck is. Thus, having a longer neck would have given a giraffe an advantage in battle, meaning that it is more likely to win and pass on the 'long neck' gene so becomes selected for. Being able to reach an abundance of previously unattainable food would have then acted as a secondary use of their long necks and would have helped to reinforce its selection.

6: Gaur: 1, 600kg/4m long

Gaur, Bos gaurus, are characterised by a very prominent dorsal hump and have a long dewlap that hangs from their chin. Other than mankind and tigers, their large size means that they are rarely preyed upon and their lives are relatively safe for a herbivore.

Gaur are typically diurnal, gregarious animals where the females live matriarchal herds with their close relatives and offspring. To avoid inbreeding, a young male leaves its herd once it has sexually matured and spends much of the year in solitude. During the breeding season, polygynous males rejoin herds where they compete for mates by showcasing their size and emitting loud calls. 

Interestingly, scientists have noticed that many gaur become nocturnal in areas with high levels of human activity. But whatever hours gaur keep, their behaviour remains the same and they seem to actively avoid water, going to sources only sparingly to drink. They rarely wash or wallow and spend much of their time browsing on a wide variety of shrubs, flowers and grasses. 

5: Hippopotamus: 2, 500kg/1.2m long (max weight: 3, 400kg)

The hide of the hippopotamus, Hippopotamus amphibius, is so thick it can make as much as a half a ton in large males! Interestingly for an animal that lives in an extremely hot environment, hippos have neither sweat nor sebaceous glands. Instead, they have unique glands that secrete a viscous red fluid. This has given rise to the urban myth that hippos 'sweat blood'.

Due to their weight, which makes them cumbersome and ungainly on land, hippos spend much of their time in water where their body weight is decreased. Hippos mainly venture onto land to feed at night, where they are able to climb even very steep banks in their search for grass. Most hippo attacks on humans happen during the night, when a person accidentally walks between the hippo and water. The hippo panics and then charges, bulling over the person as it tries to get back to safety.

Hippos are actually the most dangerous animal to humans in Africa and should be avoided if ever seen in the wild. They are extremely territorial and (in addition to what was mentioned above) may attack people on shores and knock us from boats if we venture into their 'space of water'. With hugely powerful jaws that can open almost 180 degrees, hippos are reportedly able to bite animals as big as Nile crocodiles clean in half with a single bite!

4: White rhino: 2, 350kg/3.8m long (max weight: 3, 850kg)

The white rhino, Ceratotherium simum, spends large amounts of its day wallowing in the wet mud around rivers, streams and lakes. The mud acts as a natural 'sun cream' and helps to deter parasitic insects.

Unlike their cousins, black rhinos, white rhinos are grazers and have differently shaped mouths that are adapted to trimming grass off the ground. They have poor eyesight, but have excellent hearing and even better smell. Due to this, they often walk in single file so they can follow the scent trail of the rhino ahead of them.

3: Southern elephant Seal: 3, 000kg/5m long (max weight: 4, 000kg)

There are actually two types of elephant seal, but it is the southern elephant seal, Mirounga leonina, that grows the largest. Surprisingly, their name does not stem from their enormous size, but rather from their inflatable trunk-like snouts!

Living in the brutal Antarctic waters, Southern elephant seals are superb divers and can go as deep as 1.5 miles, staying submerged for up to 2 hours at a time. While underwater, southern elephant seals hunt fish and squid, which form the staples of their diet. Both male and female seals spend months at sea at a time, where they migrate vast distances in search of food.

Elephant seals return to rookeries during their breeding season, where males compete with each in other in brutally violent displays for the right to mate with females. Successful males can form harems that consist of 40 - 50 females, whom they mate with exclusively!

2: Asian elephant: 4, 200kg/6m long (max weight: 5, 200kg)

Like their African cousins, the Asian elephant, Elephas maximus, has an extremely long gestation period and their pregnancies last for 22 months! Although this isn't surprising really, considering newborn calves can weight as much as 90kg!

Asian elephants have been domesticated by humans over thousands of years for a variety of tasks. Most often, the elephants are used to carry or move heavy objects and as taxis, where they carry people around on saddles or carriages called howdahs. Less commonly, but perhaps more famously, the elephants have been used for combat on occasion by warlords such as Hannibal, who took the elephants over the Pyrenees and the Alps in his campaign against the Roman Empire!

1: African  elephant: 8, 500kg/6.7m long (max weight: 13, 000kg)

Although African elephants are noticeably bigger than their Asian cousins, you can also use the shape of their ears to tell them apart. Rather bizarrely, the shape of an African elephant's ear looks similar to the continent of Africa, while the shape of an Indian elephant's ear looks a bit like India! 

There are actually two types of African elephant of the genus Loxodonta: the African bush elephant and the slightly smaller African forest elephant. Both are larger than the Asian elephant and can roam huge distances in search of food. The elephants eat roots, grasses, fruit and bark, which they attain using their deceptively dextile trunk and their tusks, which they use for scraping trees and digging.

Unfortunately, as with Asian elephants, their trunks have also proved to be their downfall and they were almost hunted almost to extinction by those in the ivory trade. Since the international trading ban on ivory that was placed in 1990, elephant numbers have recovered somewhat (although illegal poaching is still a problem). It's estimated that there are now as few as 700, 000 elephants left in Africa and they are officially classified as 'threatened' by the IUCN. Sadly, less than 20% of the elephants' known range is under formal protection (which is largely due to budget constraints and the instability of many Africa governments). 

The truth about panthers

If you've ever sat down and watched a nature documentary, you've probably heard the term 'big cat' bandied about. But this is not a defined scientific term and there is often some confusion with what the big cat species actually are. Generally speaking, people asked this question usually reply with these four species: lions, tigers, jaguars and leopards. All of these species belong to the genus Panthera (which are renowned for their ability of roaring) and make up the largest, heaviest families of cat.

Depending on whether panthers are atypically coloured leopards or jaguars, an adult can be 7 to 8 feet in length and can weigh between 100 and 250 pounds. They are solitary animals and are examples of an apex predator, which is an animal that has no predators in nature.

Other people may give the same list as above, but also include pumas (cougars), cheetahs and the Eurasian lynx. This is fine as well since the term is ambiguous by nature and, as I've already mentioned, isn't scientifically defined. But what is incorrect is the inclusion of the panther. Obviously panthers are quite big and, upon spotting one, you would see a big, black cat. So why aren't they on the list? The answer is simple – panthers are not actually a species of cat.

Panthers are actually just leopards or jaguars (depending on whether they live in Asia or the Americas) that have an all-black coat. Their confusing and famous colouring—which is called melanism—is simply the result of their ‘dark-coding’ allele being defective. The allele is overactive and they produce so much of the black protein melanin in their fur that it masks the cat’s normal phenotypic colouring.

This is evident upon close inspection of a panther’s fur, where you will be able to see that it is not completely black. If you look very closely, you will be able to make out the normal colouring of a leopard or jaguar (although it will be very faint). Some have even gone as far as terming this phenomenon ‘ghost striping’!

Like all mutations, this defective allele was a fluke of nature. This means that it's rarer in jaguar and leopard populations than the normal protein (although the mutation is dominant in jaguars, which is fairly unusual), so most cats simply have their normal colouring. This is true as a basic rule, although research has shown that the all-black coat is actually selected for in certain environmental conditions so the panther phenotype is slightly more prevalent there than it is normally (although it never becomes more common than the normal phenotype). The main example of this is in areas that are very dark with densely packed foliage, where the panther’s darker colouring gives it an advantage in camouflage over the rest of its kin. 

Obviously their uniform colour means that panthers are visually distinctive from their normally coloured family members, but this is the only difference between them. All other aspects of panthers, such as their size, diet and behaviour are exactly the same as normal! This knowledge—along with the fact that panthers are genetically viable with their normally coloured kin (so can breed with them successfully)—means that they aren't their own distinct species!

Round & round the straight line

If you’ve ever switched a light on in the dark you’ll have no doubt noticed the rather strange effect it has on moths, which are soon attracted to the light and begin to spiral round it for hours. Many people wonder what causes this bizarre behaviour and, at the moment, there is no definitive answer as even entomologists (the scientists who study insects) find it confusing. 

This isn’t to say that they don’t have theories regarding this behaviour however, and there is one main explanation that is generally accepted among the entomological community that seems to have some scientific evidence. This theory is surprisingly simple and basically works off the principle that lepidopterists (the family of butterflies and moths) use light to navigate when they are flying. 

Like many insects, moths have very poor vision that is mainly used just to detect light and movement.  Most of the information about their surroundings actually comes from their highly developed antennae, which provide them with an incredibly sensitive sense of smell. In fact, the antenna of male moths (pictured above) are so sensitive in some species that they can detect a single molecule of a female moth's sex pheromone in 1 cubic yard of air - allowing them to smell the moth from 11 kilometres away!

So, to start at the basics, there are two fundamental responses that all types of life (that are capable of detecting photons) have in response to light – they either respond positively to it and move towards the source (positive phototaxis) or negatively and flee from it (negative phototaxis). Lepidopterists are known to be the former, which explains why they converge on sources of light (such as bulbs). 

And while this appears to make sense so far, it is actually confusing to many scientists – why would an insect that is vulnerable to predators move towards a light source and make itself more visible? In fact, logic suggests that moths should actually show negative phototaxis and head towards the darkest areas they can find – they do have drab colours for camouflage afterall! 

However, the idea that lepidopterists use light for navigation helps to explain this and gives a plausible reason why they are in fact attracted by ambiance rather than repelled by it. The idea is simple and suggests that lepidopterists use the brightness of the lights in the sky (i.e. the sun, stars and moon) to calculate how high they are flying and use the angles that these lights hit their eyes to determine their direction. Thus, they think that because the lights are getting brighter, they are actually getting higher in the sky (which generally makes them safer from many of their predators, such as spiders, which live amidst foliage). 

So although this might seem like a bizarre explanation for why moths are attracted to light, remember that moths have evolved over millions of years in an environment where the brightness of the night sky has scarcely changed. It is only recently that humans have invented and built all of these streetlights and exterior lamps that are confusing them! Essentially, all the light we produce at night is hijacking their complex and highly evolved navigating systems because they now get much close to sources of light than they are expecting to! 

This concept also explains why moths end up spiralling round bulbs for hours at a time and, basically, because the stars and the moon are so far away from us, their light hits moth eyes in parallel to the horizontal axis of flight. Thus, moths have evolved a system where they use the information that this light provides to work out whether they are turning or travelling in a straight line. (Think of a cross where the flat line represents direction and the vertical one represents height).

This system is actually fairly simple and works well, until of course they become too close to a light. Once this happens, the angle the light strikes the eye at is steep enough to make the insect think that it is turning so it constantly has to compensate and turn itself to ‘return’ back to a straight line of flight. Thus, while we can see that the moth is actually flying in circles around the light, the disorientated moth thinks that it is flying in a straight line! 

And if this isn’t enough for the poor moths to contend with, many lepidopterists also believe that because moths are nocturnal (sleep during the day), being close to such a bright light actually makes them sleepy. When this happens, they are believed to enter a ‘rest mode’ and attempt to sleep, which is why they often try to land on (or nearby) the light – making it even harder for them to escape its clutches!

The icefish: life without respiratory pigments

Wherever you look for life on Earth, you find it. From free-floating bacteria in the upper atmosphere to tiny organisms that make their home deep within the solid crust of its surface. Animals have been found thriving in deserts hot enough to cook them alive and, similarly, have been recorded living abundantly in environments that are cold enough to freeze their bodies solid!

Living in such hostile conditions places huge strains on the organisms that live there, which, as a necessity, have had to evolve specialised physiological adaptations if they are to survive for long enough to reproduce and pass on their genes. There are many examples of extreme and unique adaptations for this purpose, with those of the Antarctic icefish being among the most bizarre.

Icefish are found throughout the cold waters that surround Antarctica and South America, belonging to the Channichthyidae family - a small group of fish that have no respiratory pigments in their blood. 

Rather uniquely among species of animals, Antarctic icefish do not rely on respiratory pigments to carry oxygen in their blood and lack them altogether! Such respiratory pigments were once believed to be essential for multicellular life to exist since they chaperone oxygen so that it can be removed from the air and absorbed by the blood, where it is at a much higher concentration than in the surrounding atmosphere. This is an important 'law' that life must overcome if it is to grow bigger than a single-celled organism because molecules naturally diffuse from an area of high concentration to one of a low concentration and not the other way around! So, simply put, without respiratory pigments the oxygen content of blood would be too low to fuel life as we know it!

Thus, the vast majority of multi-celled organisms have blood that is packed with respiratory pigments. These pigments are usually constructed from 1 of 2 key metals, with which one being used depending on an organism's evolutionary history and can be used to synthesise a number of different pigments:

• Iron, which is the most common basis of respiratory pigments, can be used to produce:
• Haemoglobin, which is found in humans and turns red when oxgenated.
• Hemethryins, which are found in terrestrial worms and brachiopods, and turn violet when oxygenated.
• Chlorocruorin, which is found in aquatic worms and turns green when oxygenated.
• Copper, which is used to make hemocyanins. Hemocyanins are blue when oxygenated and are usually found in families of molluscs and arthropods.

The fact that icefish lack such respiratory pigments is puzzling at first glance, but actually makes sense when it is considered carefully since it provides the fish with a huge advantage in their ability to survive in the harsh cold of Antarctica. Basically this advantage stems from the fact that the colder water is, the more oxygen can dissolve in it. Thus, the frigid Antarctic waters carry much more oxygen than warmer waters do so icefish breathe in more oxygen with each 'breath'. This, coupled with the fact that they have twice as much blood fluid in their body than another species of fish their size, means that they can still provide their muscles with enough oxygen to work effectively.

But why is this advantageous to their survival? Well, the fact that icefish can supply enough oxygen to their muscles in order to survive without respiratory pigments means that they do not have to make any; saving them huge amounts of energy each year that can be used for more useful tasks instead, such as feeding, reproducing and evading predators!

Furthermore having no respiratory pigments in their blood decreases its overall viscosity by about 25%, which means that their heart has to work far less hard to pump blood around their body than it otherwise would have. In addition to placing less strain on the organ so they can live for much longer before it gives out, having thinner blood also reduces their energy expenditure and allows them to move 4 times more blood which each beat than a typical fish their size - saving even more energy!

Thus, the icefish has evolved to possess one of nature's most bizarre and unprecedented adaptations that allows it to thrive in one of the most extreme environments on Earth! Without this adaptation, or another that gave it a similar advantage, the strange fish would have undoubtedly died out long ago, being unable to to survive and reproduce in the freezing waters of the Antarctic...

From black to white: is calcium really that important?

The majority of us are at ease with Darwin's concept of evolution and understand how the 'survival of the fittest' has led to the vast abundance of life on Earth. Obviously, humans are no exception to this rule and evolution has moulded us into what we are today. Evolution, for example, selected for the first of us who began to move on two legs as this freed up our hands for better tool use; and selected for those who chose to live in social groups, which provided much more protection and help than did living alone. Without evolution it is doubtful that any life would exist on Earth at all, especially not in the form of hugely sophisticated organisms like humans.

Life on Earth began sometime around 4 billion years ago. It is believed that single-celled organisms first evolved on the shores of primordial oceans, which were abundant in the resources needed for life. Over time, these cells eventually evolved into the countless forms of life that we see on Earth today.

Most of you won't be surprised by any of this; it makes sense, after all. Something you might find surprising however, is why scientists believe that the early humans settling Europe evolved from being black to white. Obviously the sun's rays are less intense in Europe than they are in Africa, meaning that European settlers wouldn't have needed to produce as much of the pigment melanin in their skin, which absorbs ultraviolet (UV) radiation. Producing less melanin then would have provided such individuals with an advantage as they wouldn't have been wasting energy producing proteins their body didn't really need. This saved energy could then have been dedicated to more important processes (like keeping warm in the colder climate, for one thing).

Although this theory makes sense logically and saving energy by producing less melanin could quite plausibly have been the difference between life and death in the harsh European winters, is it really enough to have driven the evolution of one of our most noticeable racial polymorphisms?

Many scientists believe not, at least not by itself anyway, and research into this question has provided a rather odd alternative. Simply put, many scientists now believe that Europeans evolved from having black skin to white skin due to calcium!

Calcium is an fundamental resource for our bodies, with its ions having essential roles in muscle contraction; in propagating nerve impulses; and, arguably most importantly, in forming our skeletons (via binding with phosphorous to form a very stable salt called calcium phosphate). Despite its importance, calcium is rare in nature and is extremely difficult to acquire naturally as part of our diets. As always however, Nature provided early man with an ingenious way around this and all humans are able to make vitamin D in their skin when it is exposed to sunlight (in much the same way as plants photosynthesise sugars from sunlight to use as energy). Vitamin D greatly increases the affinity of calcium absorption in the gut, allowing the body to absorb much more of any calcium that it consumed than it would otherwise be able to.

Due to this ability, most people are able to acquire enough calcium (especially during the summer) to lead normal and healthy lives, and indeed, our African ancestors would have had strong bones and efficient muscles. The problems arose however, when early explorers entered Europe where the sun's rays are much less intense. This meant that the melanin pigments in their black skin were able to absorb much more sunlight than they could while in Africa and, as a result, vitamin D could no longer be produced.

Fossil evidence suggests that it was not long before the health of these explorers deteriorated, and many adult skeletons from the period show symptoms of osteomalacia (a disease where bones soften due to lack of calcium and deform under the weight of walking), and many may have suffered from a range of muscle weakness and epileptic disorders as their reserves of calcium were depleted and less and less could be replaced from bone stripping. Obviously such ill effects greatly reduced an individual's chances of survival and those with slightly lighter skin would have been more likely to live longer. Being healthier and living longer meant that they would have been more likely to survive to reproduce and slowly, the 'lighter' genes (which produced less melanin), would have spread through the population. In each generation the palest individuals would have been most successful at surviving and breeding so, over time, European humans would have got paler and paler until their skin was as white as it is in their descendants now.

As if this selection pressure wasn't enough to drive for whiter skin, having low levels of calcium and brittle bones had another major problem for women in particular - it hindered childbirth. Many women had such brittle pelvises that they broke under the strain of labour, virtually guaranteeing that both the infant and the mother would die. Furthermore, many children suffered from severe rickets due to a lack of calcium during childhood and puberty. This meant that such individuals were physically smaller than they should have been and many women suffered from underdeveloped hips that were too narrow for a baby to pass through. As a result, such a mother and her baby would have died during labour. Thus, many of the darker individuals would have been unable to give birth so that the darker genes disappeared from the European populations very quickly - being strongly selected against by Nature!

The degree of deformity that rickets can lead to can be very extreme, almost completely debilitating a child suffering with the condition throughout their entire life.

Scientists also believe that this explains why the vast majority of Europeans (and those in their descendent colonies such as Australia and the USA) can eat dairy as a stable component of their diet. This is actually quite abnormal, both in the animal kingdom and among other ethnicities of humans, as rennin (the enzyme required to digest milk) usually stops being produced by the body in infancy after the individual has been fully weaned. Thus, most humans are lactose intolerant and experience unpleasant symptoms if they drink milk or eat too much dairy-based produce. Humans evolving in Europe however, needed as much calcium as possible and would have been under strong selection pressure to continue producing rennin throughout their lives as milk is an unrivalled source of calcium.

Thus, the importance of calcium to the human body has made it an invaluable component that we need to survive. Too little calcium leads to severe health conditions that are so extreme that they can even drive evolution into turning black humans, who have very active melanocytes (melanin-producing skin cells), into white humans who have very little sun-protective pigments in their skin (allowing them to produce more vitamin D).

Is evolution in danger of extinction?

Everyone is familiar with the concept of extinction. Sometimes species are lost forever, whether it is due to natural and unavoidable catastrophes like a large asteroid hitting our planet, which many scientists believe wiped out the dinosaurs, or due the activities of humans. The extinction of a species is sad, but the brutal fact is that it is not all bad and the death of one species allows the evolution of another to occur and fill the now empty niche. A good example is that the mass extinction of the dinosaurs allowed the previously oppressed group of mammals to evolve and become one of the most dominant forms of life on the planet, which has worked out pretty well for us... These events are called extinction spasms and follow all mass extinction events, with the 'bounce-back' time of species taking millions of years. For example, it took 20 million years after the Cretaceous Tertiary Extinction Event for the surviving marine invertebrates to establish as many new families of organisms as they'd lost.

The Barringer Crater in Arizona is 1 mile wide and 570 feet deep. It is believed to be the crash site of the city-sized KT asteroid that hit the Earth 65 million years ago with the force of million nuclear bombs, wiping out half the life-forms on the planet, including the dinosaurs.

Therefore, life has always recovered after mass extinction events and many new species have appeared after them. This is mainly because the past five mass extinction events have left many of the key environments for evolution intact, such as rainforests and underwater environments. Such environments are sometimes called 'evolutionary powerhouses' and are critical for the development of new species, having produced substantially more new species of organism than any other environment, including almost every major group of vertebrate.

The planet is currently undergoing its sixth and largest mass extinction event, which is due to the destructive activities of humans. The most damaging of our activities are mainly mass hunting and deforestation, which have resulted in many species already falling extinct. The problem with this mass extinction however, which sets it apart from the others, is that we are destroying the powerhouse environments and are killing every other category of animal at the same time, rather than just certain ones. This is resulting in a rapid loss of the planet's overall genetic diversity - diversity that is essential for life to recover after we wake up, stop destroying the planet and take steps to halt the extinction event.

An aerial view of the border between Haiti (left) and the Dominican Republic (right). The heavy logging in Haiti for the charcoal and firewood industries has resulted in mass deforestation and as a result, only 3% of Haiti's forests now remain.

As according to Charles Darwin, evolution works when a gene randomly mutates and that this mutation gives the individual an advantage over others of its species, helping it to survive for longer. Thus, the individual can breed more because it is around for a greater length of time and gradually, the mutated allele (which is naturally selected for), increases in frequency throughout the population of the species. Once a species has gone however, its genes are lost and cannot change or be passed on so their potential for evolving into a new species is also gone. New species usually evolve when separate populations of a particular species live in different environmental conditions and cannot breed with each other. Therefore, the different populations will be under different selection pressures for new genes and will undergo speciation, slowly becoming different subspecies and eventually, different species altogether.

Thus, the recovery of life after a mass extinction event depends upon the species that survive it and the genes that remain - genes cannot just appear from nowhere! Therefore the rapid loss of forested habitats, which have survived remarkably well in past mass extinctions, is greatly reducing the planet's genetic 'resource base' and is pushing even more species extinct. Worryingly, it is looking more and more likely that the process of evolution will become severely limited in its capacity to create new species of life and ultimately, may fail and become extinct itself. If this should happen life on Earth will die (eventually humans will die out as well as we'll have no food) and will not be replaced, leaving the planet as just another barren and lifeless rock drifting through space...