Columbia mammoths, which were relatives of woolly mammoths, roamed as far south as the Black Rock Desert in Nevada, where this fossil was found. The fossil was excavated under the direction of Stephanie D. Livingston, then of the Desert Research Institute

People have been fascinated by woolly mammoths for a long time. Before people even knew how to grow crops or make things from metal, they were decorating their walls with pictures of mammoths. Scientists have found an ancient figurine of a mammoth that was carved from a mammoth tusk in Germany 35,000 years ago. And in a cave in France around the same time, someone drew a picture of a mammoth.

But over time, people stopped drawing mammoths because no more were left. The last known mammoths lived on Wrangel Island in Siberia 3,700 years ago, at the end of a time period called the Pleistocene epoch.

Scientists have long wondered why the mammoths disappeared. Cave paintings of mammoth hunts make it clear that humans killed some of the giant animals, but now scientists are learning that different groups of mammoths went extinct at different times in different places. And it took more than just people to kill them off — changes in climate and maybe even a comet from space were also to blame.

The kind of woolly mammoths that people drew was as tall as an elephant and lived in big groups. They roamed cold northern regions around the globe from about 150,000 years ago until they went extinct. They had three inches of fat beneath their skin to keep them warm. Long, shaggy hair helped keep them warm, too. The Northern Hemisphere during the age of mammoths was covered in steppe-tundra, which is a cold, dry habitat containing scattered patches of herbs, shrubs, scrubby trees and grasses, all of which the mammoths ate. Their tusks, bigger and more curved than those on elephants, probably helped them move snow to find food. When a person stood near a mammoth, could we blame them for awe? No wonder people drew mammoth pictures on cave walls.

But people were thinking about mammoths as more than models for art. Archaeologists—scientists who study ancient cultures by looking at what they leave behind—have discovered that people used mammoth bones to make tools. They used mammoth ribs to build houses. People probably ate mammoth meat and wore mammoth skins. Some people depended on mammoths for their survival.

As humans spread across Europe and Asia 40,000 years ago, they increased in number and honed their hunting abilities. During the same period (from 40,000 years ago to 3,700 years ago), mammoth populations declined and then went extinct. Beginning in 1860, scientists argued that mammoths vanished because people hunted them to extinction. Over-hunting was also blamed for the extinction of woolly rhinoceros, mastodons, cave bears and other mammals. This idea was so common that some people called it “the favorite hypothesis.”

A hypothesis is a proposal that predicts cause and effect. The “favorite hypothesis” suggested that human hunting caused mammoth extinction—but it wasn’t the only hypothesis explaining the mammoth’s demise.

In the 1800′s, scientists didn’t know what the climate was like when mammoths roamed the
Earth, but that has changed. Today, scientists look at records from deep-sea drilling and ice cores from Antarctica and Greenland. By studying the minerals and gases that were trapped in these cores long ago, they can tell what the climate was like in the past. They also look at pollen and plant fossils from all over the world to see what kinds of plants were able to live in different places in the past. Scientists now know when the mammoth’s world was cold (about 8˚C, or 46˚F, in the summer months), dry, and covered with ice sheets. They also know when it was a bit warmer (about 15˚C, or 59˚F, in the summer months) and wetter.

The steppe-tundra habitat favored by mammoths doesn’t exist today. Because woolly mammoths went extinct when ice sheets became rare and the world became warm and wet, some scientists hypothesized that changes in long-term patterns of weather, called climate change, caused mammoths to die.

Last year, a group of scientists from the Museo Nacional de Ciencias Naturales in Madrid, Spain wanted to know what killed the mammoths—and unlike scientists before them, they had data on how much steppe-tundra existed over long periods of time. They also had data on where woolly mammoth bodies had been found, and where people lived between 126,000 years ago and 6,000 years ago. Even though the human populations increased at the same time that the climate warmed and the steppe-tundra habitat vanished, the scientists were able to determine which change caused the mammoth extinction.

Twice in the history of mammoths, their steppe-tundra homes almost disappeared. The first time it happened, starting 126,000 years ago, mammoths nearly went extinct—but they recovered. The second time it happened, starting 6,000 years ago, mammoths actually went extinct. How come the mammoths died off the second time but not the first?

The first time the steppe-tundra habitats receded, humans were not fully modern. They did not have art or advanced tools. They probably couldn’t hunt something as big and intelligent as a woolly mammoth, the scientists think. Humans didn’t have the technology to survive in the very cold places woolly mammoths liked best.

By the second time steppe-tundra habitats receded , humans had changed. Through the use of more complex clothing and housing, people could survive in the cold climates favored by mammoths. Also, people had invented complicated tools, like spear-throwing devices, which allowed them to hunt more dangerous animals. We know people hunted mammoths because mammoth bones have been found with spear holes in them.

Woolly mammoths marching across the steppe-tundra.

Mauricio Anton

The scientists from Spain suggest that climate change pushed woolly mammoths into smaller and smaller patches of steppe-tundra habitat. Humans, with their increasing numbers and new hunting technologies, finished them off.

That explanation may be satisfying, but there’s a problem, or at least a complication: mammoths weren’t the only species to go extinct during that period. At least 35 kinds of North American mammals, including camels, ground sloths, horses and mastodons, died off along with the mammoths.

Scientists have a hard time imagining that human hunters killed everything. According to Donald Grayson, an anthropologist at the University of Washington in Seattle, “One huge problem with the idea that people hunted all the late Ice Age North American mammals to extinction is that although at least 35 kinds of large mammals became extinct at this time, we can only show that humans hunted two of them—mammoths and mastodons.”

Because the over-hunting hypotheses could not be the only explanation, a group of 26 scientists from all over the world looked to the sky. Could a comet have changed North America and killed the mammals?

Richard Firestone, lead scientist and chemist from Lawrence Berkeley National Laboratory in California, describes one possible scenario: “When the comet exploded in the air over the Great Lakes, there was a loud roar, a flash of light and heat hotter than the sun. A great wind of dust and debris followed, rolling across North America to the Atlantic and Pacific Oceans. It churned up the landscape and choked the animals. Hot ash fell across the land, causing vast forest and grass fires. The sky went dark for months until the dust settled.”

Led by Firestone, the team of scientists looked for evidence of a comet. Impacts from comets often are associated with tiny diamonds called nanodiamonds. Comets can bring these diamonds to Earth, and the heat caused by impact can also change graphite in the earth into nanodiamonds. The scientists believe they found nanodiamonds caused by an impact around 12,500 years ago. Such an impact might have helped shrink the amount of steppe-tundra available for mammoths.

Like many hypotheses about extinctions, the comet hypothesis doesn’t explain everything. If a comet exploded in North America, and it was big enough to cause global climate change, then the impact crater should be visible. None is. Of course, that doesn’t mean some intrepid explorer won’t find one.

Most likely, no one thing alone wiped out all of the mammoths. Perhaps a comet destroyed some of the mammoth’s habitat, and hunters killed many of the animals that were left. A small number of mammoths held on in other areas, like on Wrangel Island, only to eventually die off later

People were thinking about woolly mammoths 30,000 years ago, and they are still thinking about them now. People will probably be thinking about woolly mammoths for years to come. The icy, dry world filled with hairy giants has melted away, but it lives on in laboratories—and our imaginations.

Word Find: Woolly Mammoths

The platypus is one of two kinds of mammals that lay eggs instead of giving birth to live young. Scientists are studying these creatures to better understand how reptiles gave rise to mammals.

The first European scientist who saw a platypus thought it was a fake.

In the late 18th century, British scientist George Shaw received a package from the governor of Australia. Shaw found strange things when he opened the box. He found a preserved pelt of chocolate brown. The face on the pelt looked like a rodent’s, but with a duckbill. Instead of paws, the creature had ducklike feet. Looking at all this, Shaw thought someone had sewn duck parts onto a beaver as a joke.

Of course, the platypus is real. “They’re cute and funny at the same time,” says genome scientist Wesley Warren about platypuses. He works at Washington University in St. Louis. “That’s why they’re so appealing to so many people.”

With its rich fur and funny bill and feet, the platypus looks cuddly. But don’t be fooled. Male platypuses make poison that, when they feel threatened, can be injected through sharp spurs on the hind feet. If the spurs stab you, you won’t die. You will, however, be in a lot of pain.

Poison and duck feet aren’t the platypus’s only crazy features. Female platypuses lay eggs — only one of two mammals that do so. What if your dog laid eggs? And while platypuses nurse their babies like all other mammals, they don’t do so through nipples. Instead, young platypuses lap milk from grooves in their mother’s belly.

The weirdness doesn’t stop there. Platypuses also have special organs called electroreceptors, which are used to sense a platypus’s surroundings when it swims. Electroreceptors both create electric fields and detect changes in those fields. The organs let platypuses find food in murky water. While very few mammals have these organs, many primitive fish (like sharks and rays) do.

“There’s a very unique biology associated with the platypus,” Warren said, and he might have understated his case.

Platypuses have bills and webbed feet like a duck, and also have egg-laying in common with the birds. But fur and young that nurse are mammalian features.

Doctor_bass / iStockphoto

In the platypus, nature has put familiar characteristics together in an unfamiliar way. Fur and milk are characteristic of mammals, but where are platypuses’ nipples? Some fish have electroreceptors, as do echidnas, the other egg-laying mammal. Birds have beaks, eggs and webbed feet. Reptiles make poison and lay eggs, too.

Because platypuses have reptilian features, they’re the perfect study subjects for scientists who wonder how reptiles gave rise to mammals about 270 million years ago.

Recently, Warren and his colleague Richard Wilson, also at Washington University, put together a team of 102 scientists to study the platypus genome. A genome is an individual blueprint, like a map or a plan, for a creature. An organism’s genome has all its hereditary information. And that information is encoded, like a secret message, in a long molecule called DNA.

Scientists from universities and research institutes all over the world worked on this platypus genome project. Warren and Wilson split the DNA decoding among a number of specialized groups.

Like reptiles, platypuses lay eggs and make poison. Spurs on the hind feet of platypuses deliver the toxin when the mammals feel threatened.

stowe boyd / flickr

One group studied fragments of DNA that move around within the genome of a single cell. Another group examined very short fragments of DNA that repeat themselves. Another studied active portions of the genome, those parts that code for a message, while yet another group studied the inactive portions of the genome. All of these groups have multiple scientists, so the project is made up of a lot of scientists.

With so many scientists cooperating, you might think they’d produce an encyclopedia of knowledge. But examining the genome of one animal, even one as odd as the platypus, doesn’t tell you very much. Eric Lander at the Broad Institute in Cambridge, Mass., explained it this way: “If you really want to understand a genome, you have to compare it to other genomes.”

After they decoded the platypus DNA, Warren and his colleagues compared the genome to other creatures. But which other creatures?

Choices are limited. Scientists have decoded the DNA of roughly 1,000 creatures, but many of those are bacteria and viruses. Approximately 40 mammals have had their genome mapped. Of the birds, only the chicken has had its genome decoded.

“And for reptiles,” says Warren, “there’s the turtle genome we’re sequencing now, and the green anole lizard was recently sequenced. That’s it.”

Why have scientists mapped so few complex organisms? The answer is time and money. The first complex organism to have its DNA decoded was a roundworm called C. elegans. It took eight years to complete that decoding. The platypus genome project was completed in only four months, but it had the benefit of information from previous genome decodings and cost nine million dollars. That’s a lot of money.

Both the time and money needed to complete genome projects are decreasing as technology improves.

“In the future, you’ll be able to sequence a genome of platypus size for maybe a million dollars in two months,” says Warren.

Echidnas are egg-laying mammals, like platypuses. Platypuses live in freshwater in Australia; echidnas are terrestrial and live in Australia and the large island of New Guinea.

For now, scientists who want to understand platypus evolution (or how, over millions of years, the creature came to be the strange mammal that it is) have only a few species available for comparison. Still, scientists were able to learn a lot with what they had. They compared the platypus genome to that of humans, the anole lizard and the chicken. This allowed them to examine the platypus’ mammalian features (lactation, or milk production), its reptilian features (electroreceptors, poison and some aspects of egg laying) and its avian features (the duckbill and feet, and some features of egg laying).

The first mammals gave rise to two groups of mammals 166 million years ago. One group includes platypuses and echidnas, which look like a pincushion. The other group includes humans. By comparing the genomes of humans and platypuses, scientists found that both species have genes to make the same kind of milk, even though platypuses don’t have nipples. Because both species make milk using the same gene, scientists know that mammals have been making milk for more than 166 million years, which is when humans and platypuses last had an ancestor in common. So, this common ancestor also made milk.

The scientists also looked at genes that make platypus electroreceptors. These genes aren’t found in mammals or lizards. This means that platypuses didn’t inherit electroreceptors from some distant relative. And the scientists also found that although many reptiles make poisons, they don’t use the same genes as platypuses do to make their poisons. This means the platypus evolved unique genes for making both electroreceptors and poison.

Scientists also compared genes from the platypus to those of chickens and humans to understand the evolution of egg formation. Scientists found genetic material in common with platypuses and humans, but not with chickens. This explains some similarities between human eggs and platypus eggs. For example, both species have a membrane surrounding the egg.

The platypus genome project also found that platypuses, chickens and lizards share genes that humans lack. For example, lizards and chickens have three genes that help them make egg yolks, platypuses have one of these genes and humans have none of these genes. This number of egg-yolk genes tells us about how animals evolved ways to get nutrients to their young. With three genes to produce egg yolk and no genes to produce milk, chickens and lizards (both of which split from mammal-like reptiles about 315 million years ago) deliver nutrition to their young wholly from egg yolk. With milk-producing genes and one egg-yolk gene, platypuses rely more on milk than yolk — but they still use yolk. Humans, who have milk genes but no genes for yolk, deliver their nutrition to their young with milk.

Scientists have discovered that in the case of the platypus, looks aren’t deceiving. Platypuses really are a mixture of lizard, bird and mammal — with a dash of something uniquely platypus tossed in.

“It makes you scratch your head and wonder how nature could let this happen,” Warren said. And, “how all these ancient reptilian structures could survive.”

Scientists still don’t know what all platypus genes do. When they figure out more about the mystery genes, scientists will understand the way mammals evolved even better than researchers do today. Did the very first platypuses have electroreceptors like sharks do, or did platypuses evolve these organs later? Someday, scientists like the ones studying the platypus genome might be able to tell us the answer.

And what’s Wesley Warren going to do now that he’s finished this huge platypus project? He’s helping figure out the genomes of other animals — the zebra finch, a common striped bird with a bright red beak, and the western painted turtle.

Word Find: Puzzling Platypus