The mammals inherit the earth: how a family of nocturnal forest-dwellers became the next masters of life on the land
Monday 09 February 2009
Had that asteroid missed the Earth 65.5 million years ago, would many of the animals we know and love today, including mankind, ever have evolved?
No one can possibly know the answer because life on Earth cannot be re-run through a computer programme to see what would have happened if the dinosaurs had survived.
What is clear is that evolution took a dramatically different path following their extinction. The disappearance of the dinosaurs provided huge new opportunities for another group of animals to take centre stage and become the next masters of the Earth. When the dinosaurs were in charge, mammals clung on at the edge of the world's families of living creatures.
Mammals back then were mostly small and squirrel-like. They emerged from their underground burrows only when it was safe to do so, often at night. When the coast was clear, they'd scuttle off to feed on insects – easy food for ground dwellers or those who chose to hide out of harm's way safely tucked up in a tall tree.
Many of them developed evolutionary tricks to help survive the terrifying dinosaurs. Most gave birth to live offspring, so they didn't leave eggs lying around as food for someone else's breakfast. They were so scared of leaving their burrows that they developed their own personal soup kitchens, in the form of breast milk, so they could feed their young at any time, day or night, without leaving the nest. They also grew fur which helped them to keep warm. Finally, warm-bloodedness let them hunt at night when it was cold outside, and they would be safer from predatory dinosaurs.
So, when disaster struck 65.5 million years ago, these small, furry mammals were well placed to survive the impact and the dreadful blackout that followed. Once the dust had settled, mammals could enjoy the sunshine without the threat of being eaten by reptilian monsters. Birds were now the only living branch of the dinosaurs left, filling the world with song as the Earth moved into a fresh new dawn.
Things got off to a quick start. Within 3 million years shrew-sized mammals had evolved into creatures as large as dogs. Within 5 million years, mammals of all shapes and sizes roamed the land. With the dinosaurs gone the land was a place of relative peace and tranquillity and nature adapted itself to fill every available niche for new life.
This epoch, from 56 to 34 million years ago, is called the Eocene Period ( see the geological column in Part 1). The name comes from the ancient Greek word meaning "new dawn". Extraordinary mammals filled every available corner. In the forests there roamed deer, boars, bears, koalas, pandas, monkeys and apes. On the grasslands grazed cattle, bison, oxen, sheep, zebra, pigs, horses, giraffes, kangaroos and donkeys. In burrows dwelt rabbits, badgers, hedgehogs, mice and foxes. In rivers and swamps splashed rhinos, elephants, beavers, otters and hippos. In the deserts trekked camels, llamas and rats. In the oceans swam whales, dolphins, seals, walruses. And in the air flew bats.
Mammals evolved into three main groups. Two of them, including nearly all of today's mammals, originated in Laurasia, the northern fragment of Pangaea, and one from the southern part, Gondwana. It is only the monotremes that survive from Gondwana. They are a very strange lot. The most famous of them all is very strange indeed.
Is it a duck? Or perhaps a weird fish? Or maybe a bird? Or even a beaver? The question baffled experts when the creature was first discovered by Europeans in Australia in the late 1700s. To begin with, the duckbilled platypus was dismissed as a hoax, a kind of half-bird, half-mammal. Surely someone was having a laugh: naturalists suspected that perhaps it was a duck's beak sewn on to the body of a beaver – they even tried to find the stitches with a pair of scissors. But by 1800 people accepted that this was something completely new and unknown – but most definitely natural.
Duckbills are mammals because, even though they lay eggs, their babies are fed by milk produced by their mother's mammary glands. Platypus fossils dating back 110 million years show that these early mammals originally waddled over to Australia from South America before the two landmasses fully broke up into separate island continents.
The name means "pouch" in Latin. These mammals rear their young in a special pouch on their outer skin. They give birth to extremely tiny babies which crawl up the mother's belly and tuck themselves inside her pouch, latching onto her nipple for food and drink. There they stay for a number of weeks until they are big enough to leave the pouch on their own, returning for warmth and food as and when they want.
Marsupial species developed in similar ways to other non-marsupial mammals that we are familiar with today. There is still a marsupial version of the mole, and a marsupial mouse. Once there were marsupial cats, dogs and lions on the Australian continent, too.
But their luck began to change with the arrival, about 15 million years ago, of bats and rats which came over from Asia as it drifted close enough for them to fly or raft across the sea. Then, from about 40,000 years ago, people arrived in canoes. In time they would introduce dingoes, rabbits, camels, horses and foxes (for hunting), all of which have undermined the native ecosystem for indigenous Australian and Tasmanian marsupials such as the Tasmanian Tiger, the last of which died in Hobart Zoo in 1936. Shortly before her death a black-and-white film was made of her pacing up and down in her enclosure. As a final farewell, she bit the cameraman on the backside.
The third, and by far the biggest, group of mammals is the placentals. These creatures (to which humans belong) keep their babies inside their bodies until they are well developed. The baby's blood and the mother's blood are brought into close contact by a food-exchange organ called the placenta, through which nutrients pass from the mother to the baby, and waste passes from the baby to the mother's blood.
The first placentals to evolve, the afrotheres, were probably relatives of today's elephants – not that they looked like elephants, since all mammals in the early days were small, furry, and fed at night for fear of being eaten by dinosaurs. About 50 million years ago, several members of the same family as the elephants returned to the sea. These are the sea cows. They use their front feet for swimming underwater and their tail as a rudder for steering.
Ungulates are mammals that evolved independently about 54 million years ago in Africa. They have survived to become today's deer, sheep, antelopes, pigs, goats, cattle, giraffes and hippos. Some of them also returned to the water. Today's whales and dolphins are actually related to ancient seafaring hippos that lost contact with the land, shedding their legs in a process of evolutionary "tidying up".
Meanwhile, in North America (not yet connected to its southern counterpart), other types of ungulates evolved. Their descendants are today's horses, camels and rhinos. They include the Hyracotherium, a tiny horse only about the size of a small dog. Originally, these creatures lived in forests, but as the climate dried out, from about 35 million years ago, many forests were replaced by open grasslands.
Open spaces meant that small horses were more exposed. Only those with longer legs, those that could gallop more successfully out of harm's way, survived. So, over millions of years, the Hyracotherium evolved into today's much larger and faster steeds.
Bizarre as it sounds, camels also originated in North America. They eventually spread to other parts of the world, along with horses, crossing a land bridge from Alaska into Asia and then into Mongolia, finally reaching the Asian steppes and the deserts of the Middle East by about two and a half million years ago.
Next came paws. Carnivora is a mammal family which evolved about 42 million years ago and whose living descendants include cats (for example lions and cheetahs), dogs (such as wolves and jackals), weasels, bears (including the panda), hyenas, seals, sea lions and walruses.
Shortly after the paws came claws. These are the rodents. Today's descendants include mice, rats, rabbits, hares, gerbils, voles, hamsters, beavers, squirrels, marmots and guinea pigs. They are notorious for spreading disease such as plague, including the dreaded Black Death ( see Part 11). In South America, some rodents grew to an enormous size, such as the giant capybara ( Protohydrochoerus), which grew as large as a donkey.
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