Human Extinctions: The First Waves

The first rumblings of the latest mass extinctions began around 50,000 years ago. Up until that time, Australia was home to a collection of giants, including 1‑ton wombats, kangaroos standing over 10 feet tall, marsupial “lions,” and 30‑foot lizards. Fossils in Australia are scarce, so it is difficult to know precisely when most species disappeared. But one species, a 200‑pound flightless bird called Genyornis, left behind thousands of eggshell fragments. The eggshells promptly disappeared 50,000 years ago. And around that time a new species arrived on Australia’s shores: humans.

This same series of events–humans arrive and big animals go extinct–was later repeated many times around the world. The oldest evidence of humans in the New World comes from a site in Chile called Monte Verde that’s been dated to 14,700 years ago. Archaeologists are working on other sites that may be a few thousand years older still. These early colonizers may have traveled down North and South America by boat. Before about 12,000 years ago it simply would have been impossible for them to migrate on foot from Alaska, because the land was still cloaked in glaciers. After the glaciers had retreated, a new culture appeared in North America, bringing with it spears that could bring down a mastodon. And by around 11,000 years ago, the New World was stripped clean of its mastodons, its giant ground sloths, and just about every other mammal species over 100 pounds.

About 2,000 years ago travelers from southeast Asia landed on the shores of Madagascar. There they encountered the elephant bird, a flightless creature that weighed more than 1,000 pounds, and giant lemurs the size of gorillas. Neither lived more than a few centuries alongside humans. Until the 1300s New Zealand was home to 11 species of moa, another giant flightless bird. Although not quite as heavy as elephant birds, they would have overshadowed them at a height of 12 feet. Like the elephant birds, they lasted a few hundred years after the arrival of humans.

The sudden extinctions in North America were the first ones to be discovered, and initially many paleontologists thought they were caused by the end of the Ice Age. As the climate warmed, they argued, North American trees and grasses shifted their ranges, and the mammals that depended on them couldn’t handle the sudden rearrangement. But with more research, the connection between climate change and extinction seems to be little more than coincidence. If the end of the last Ice Age was so devastating to North America, you’d expect that the ends of previous ice ages would have had a similar effect. Yet during the last million years, mammals in North America actually experienced relatively few extinctions. Despite the advance and retreat of glaciers a mile thick every 100,000 years or so, mammals have managed to endure by shifting their ranges with the shifting trees and grasses. And climatically speaking, there was nothing unusual about the end of the last Ice Age in North America compared to all the other ice ages. Casting more doubt on the climate‑extinction link is the fact that while mammals were suddenly disappearing in North America 12,000 years ago, equally drastic climate changes happening in Europe, Africa, or Asia caused no significant extinctions. The one unusual thing about the end of the last ice age in North America was that it saw the arrival of humans. And as scientists have uncovered the histories of places like Australia, Madagascar, and New Zealand, they’ve found evidence suggesting that the big mammals and birds also went suddenly extinct soon after humans arrived–in some cases long before the end of the Ice Age and in others cases long after.

Humans may have brought about many of these extinctions with little more than spears and arrows. The ancestors of humans became hunters in Africa and spread out gradually into Europe and Asia. Over hundreds of thousands of years, the animals that they hunted had time to adapt to this new threat. But around 50,000 years ago, modern humans began moving quickly to continents and islands where humans had never been seen before. When experienced hunters arrived in Australia, North America, and elsewhere, they encountered animals unprepared for their assault. Most vulnerable would have been big, slow animals that couldn’t reproduce quickly.

The extinction of big herbivores may have changed the landscapes of entire continents. According to Tim Flannery, an Australian zoologist at Harvard University, the disappearance of Australia’s grazing wombats and kangaroos helped uneaten vegetation pile up on the forest floors. When lightning struck, it ignited this fuel and started enormous fires where none could have burned before. The plants that had dominated Australia before humans arrived, such as southern pine and tree ferns, couldn’t protect themselves well against fire; they lost ground to fire‑tolerant species like eucalyptus. Now they can be found in only a few rare pockets in the outback.

The old Australian rain forests had soaked up water vapor like sponges, making the climate of Australia much moister than it is today, with flowing rivers and brimming lakes that could support pelicans, cormorants, and other birds. The eucalyptus trees that have replaced the jungles can’t hold on to much water, so the lakes and rivers dried up. The leaf‑browsing mammals that hadn’t been wiped out in the first wave of hunting would have faced a starkly changed habitat, with nothing but low‑nutrient eucalyptus trees and shrubs to feed on. The only marsupials that survived were fast, like red kangaroos, or lived in eucalyptus trees, like koalas, or could hide in burrows, like wombats. Any species that was not already preadapted to life with humans, according to Flannery, was doomed.

History in a Hole

Paleontologists who study extinctions millions of years ago are happy if they can prove that a die‑off took less than a few thousand years to take place. But for those who study the current mass extinction, it’s sometimes possible to bring that resolution down to decades, even years. All that scientists have to do is find the right place to dig.

One of those right places is a cave in Hawaii. David Burney, a paleoecologist from Fordham University in New York, has been digging a hole in it since 1997. A few years earlier Burney had been wandering with a team of scientists around the south coast of Kauai, one of the westernmost islands in the Hawaiian archipelago. They were looking for fossils and other traces of extinct life when they came across the narrow mouth of a limestone cave known locally as Mahaulepu. They squeezed through, into a corridor decorated by stalactites and flowstones. After 50 feet, they emerged into an arena of sunlight and trees. They were in what had once been a high gallery in the cave, the roof of which had collapsed thousands of years ago. Seeds had blown down over the 50‑foot‑high walls and sprouted, creating a sunken garden. Burney stopped, assembled a long metal tube he had been carrying, and drove it into the silty soil. When he drew it back up, he found a fossil inside: the fragile skull of a coot, a native Hawaiian bird. Here, Burney decided, he would dig a deep hole.

He mostly used his hands to dig, for fear of shattering delicate fossils with a shovel blade. Below the top few feet of silty soil, he reached black peat, and after a few more feet, the water table. From then on, sump pumps kept the hole dry for him; when he turned them off at the end of each day, the hole filled with water in a matter of minutes. Burney put the dirt in buckets, which he brought up to the surface. Volunteers washed the buckets through screens in inflatable children’s swimming pools. A team of experts then sorted and bagged the fossils that were left behind and sent them to museums and labs where they could be studied closely. Burney also took samples of the soil, which he later searched for spores and pollen to determine what sort of plants grew around the cave.

The hole is now 20 feet deep and 40 feet across. The carbon isotopes in the bits of plant material at the bottom of the hole put its age at 10,000 years. For 3,000 years an underground stream slowly dropped silt on the cave floor. A rise in sea level let the ocean invade 7,000 years ago, whereupon the roof collapsed. A shallow freshwater pond formed on top of the denser saltwater that saturated the ground. Animals and plants fell over the cave walls into the pond and sank into the muck at the bottom. Caves are good for preserving bones, while lakes are good for preserving pollen from plants. A lake in a cave, like the one in Mahaulepu, is the best of all. Burney’s hole provides a unified 10,000‑year history of Hawaii, something no one had ever found before. He calls it “my poor‑man’s time machine.” It is also one of the starkest examples of how humans trigger waves of extinctions.

Many of the plants and animals that Burney found at the bottom of his hole were unique to Hawaii. It is not easy for life to get to Hawaii, since it is 2,300 miles from the nearest continent. A hard‑shelled seed may wheel through the ocean’s gyres until it arrives on Hawaii’s shores. Birds and bats blown off course sometimes settle on the islands, and migratory birds use them as a stopover on their way north or south. Sometimes on their muddy feet they carry a snail’s egg or a fern spore, which can then establish themselves on the island.

The pollen and seeds that Burney has found at the bottom of his hole came from a lush coastal forest of palms, mimosa‑like shrubs, and ferns. When animals arrived in the forest, they faced no mammal predators and few competitors for this lush wealth of food. Just as the finches of the Galápagos and the cichlids of Lake Victoria diversified, so did life on Hawaii. One or two species of fruit flies arrived 30 million years ago and have since evolved into an estimated 1,000 species, none of which is found anywhere else. Two dozen or so species of terrestrial snails came to Hawaii and exploded into more than 700 species. The snails probably supported the huge population of land crabs that prowled the forest floors, whose bodies Burney finds entombed in the cave.

Animals–especially birds–evolved to take advantage of every possible niche on the islands. Most of the world’s owls hunt rodents and other small animals on the ground; Burney has found the skeletons of owls that had evolved to be more like hawks, grabbing other birds in flight. Three million years ago a single finch flew to Hawaii from North America and gave rise to 100 species of honeycreepers. Burney finds skulls of honeycreepers with massive nutcracker‑like beaks that can open seeds too hard for any other animal on Hawaii to eat. Another bird species, the iiwi, uses a curved beak as delicate as an eyedropper to draw nectar from flowers.

The island of Kauai was created by volcanoes only 5 million years ago; in that time, some of its birds evolved into avian versions of pigs and goats. “Ducks and geese out here in the Hawaiian islands actually had the opportunity to become completely terrestrial animals,” Burney explains, “to become much larger than they would ordinarily; to stop flying–they didn’t need to fly; and to begin to be grazers and browsers. Among the extinct ducks and geese of these islands, birds were in a sense reinventing the goat or the pig.” The ducks lost their wings and grew to the size of turkeys, with turtle‑like beaks they used to crop grass. The geese lost their wings as well and grew to twice the size of today’s Canada goose. Some waterfowl evolved toothlike ridges on their beaks to strip ferns clean.

At the bottom of his hole, Burney finds 45 species of birds and 14 species of land snails, in addition to other species such as bats and crabs. As Burney moves up the walls of the hole, moving up through time, he finds signs of the occasional natural disruption, such as an intrusion of the ocean marked by the bones of a mullet. But otherwise the birds, snails, crabs, palm trees, mimosas, and other species native to Kauai continue to leave behind their fossils. The land snails are particularly abundant, with more than a thousand shells in every liter of mud. For thousands of years, the fossil record is pretty much the same.

Then, about 900 years ago, a new fossil appears in the hole: a rat.

Rats arrived in Hawaii with the first Polynesian settlers who came to the islands about 1,000 years ago. The next few centuries are a blur in Burney’s hole thanks to a tsunami that surged into the cave around 1500 A.D., washing away several feet of accumulated muck. But although it may have robbed Burney of a portion of history, it made up for its crime by bringing with it manmade objects. Burney finds fishhooks made of bones, and the sea urchin spines Hawaiians used to sharpen them. He has found a disk of glass‑smooth basalt, which, when wet, serves as a mirror. There are tattoo needles and fragments of canoe paddles and bottle gourds with painted designs. The traces of new plants also appear for the first time in this layer, species such as bitter yam and coconut, which Polynesians brought with them to Hawaii. The bones of chickens, dogs, and pigs–all brought by the first Hawaiians as well–also turn up.

It is at this point in Burney’s hole that native species start disappearing. Native land snails, once so abundant, begin to thin out. Palms and other forest trees no longer leave their pollen. The land crabs dwindle. The big grazing and browsing birds vanish. The long‑legged owl disappears, replaced by the short‑eared owl, which thrived on the island by eating the newly arrived rats.

In 1778 Captain Cook was the first European to visit the Hawaiian Islands, and the first place he landed was just a few miles away from Mahaulepu. As a gift to the king of the island he offered a pair of goats. Burney can find the bones of those goats’ descendants in his hole, as well as of other European immigrants: animals such as horse and sheep, and plants such as Java plum and mesquite. Cattle, which grazed on the land surrounding the cave in the late 1800s, leave their bones behind as well. Giant cane toads and rosy wolf snails, brought to Hawaii this century to control pests, leave fossils in abundance.

Burney can find only a few native species in the hole after Cook’s arrival. Below that mark, he has found 14 native species of land snails by the thousands; in the top few feet of the hole, he finds not one of them. Of the birds, only the migrant shorebirds and seabirds remain alive today near the cave. A few other species of birds that left fossils deep in the hole are still alive but survive only in remote mountain forests. Likewise, the plants that made up the forests are either extinct or hiding in refuges. The mimosa‑like shrub Burney found, one of the dominant plants in the hole for thousands of years, can be found today only on a single rock off the island of Kahoolawe. Only two bushes remain alive.

The picture in Burney’s hole is painfully clear: humans come, native wildlife goes.