Oxygen gradually accumulated on earth from the photosynthesis of plants. Once oxygen reached critical mass, changes were sudden. If you look at the paleontological record in the soil, there is evidence of oxygen-free microbes in one layer, followed closely by oxygen-dependent microbes in another layer. This introduction of oxygen, though a boon to most life, spelled destruction for a good deal of earth’s early ancestors who excelled without it.
Oxygen made the planet livable. Once established, oxygen patrolled the atmosphere capturing all the hydrogen atoms trying to get away and turned them back into water and rain. Now an ozone shield could form, dampening the intensity of ultraviolet light. All plants and animals depend on oxygen as part of their life cycles, lonely exceptions being the microscopic nematode worms that get along in the stagnant oxygen-free depths of the Black Sea and the creatures that survive on those deep-ocean geysers.
The beginning of animal life had to wait about four billion years until atmospheric oxygen began to rise toward present levels. According to Andrew Knoll, complex multicellular organisms and oxygen first appeared in the fossil record some 580 million to 560 million years ago, during the Ediacaran period. “The oxygen increase pushed earth toward its present state, but it didn’t achieve it all in one go,” he told me when I visited with him at Harvard.
Life didn’t burst forth onto center stage in full and varied forms until the Cambrian period from 542 million to 488 million years ago.
The Burgess Shale, the famous quarry of Cambrian life discovered in 1909 by Charles Walcott, a paleontologist and former director of the Smithsonian Institution, sits high in the Canadian Rockies on the eastern border of British Columbia. It is perched at about eight thousand feet on the western slope of a ridge connecting Mount Field and Mount Wapta in Yoho National Park, near the tourist destinations of Banff and Lake Louise. The view from the rocky slopes of the Walcott Quarry—surrounded by a thick conifer forest, Emerald Lake below, and the snowcapped Canadian Rockies beyond—is one of the finest on the continent. Walcott’s daughter, Helen, wrote to her brother Benjamin in March 1912 when she was touring Europe, describing castles, fortresses, the Appian Way, and the Roman aqueducts, “but I’d prefer Burgess Pass to anything I’ve seen yet,” she said.
Our first really good display of what nature was up to during the Cambrian explosion didn’t materialize until 530 million years ago, when mudslides at the Walcott Quarry captured a broad selection of fossil samples reflecting much of the Cambrian’s incredible animal diversity. Among paleontologists, the stature of these finds can only be appreciated when you take into consideration that since this geological period, and over a vast range of time in which life has been through enormous changes and upheavals, no new body designs, no new phyla, have been added to the collection of life displayed at the Burgess.
The Burgess Shale is a miracle of preservation. Stephen Jay Gould proclaimed in his book Wonderful Life: The Burgess Shale and the Nature of History that mammalian evolution “is a tale told by teeth mating to produce slightly altered descendant teeth.” Which is to say that if it weren’t for teeth, we wouldn’t know as much about our ancestors. Teeth outlast everything else and are the dominant feature in any anthropological collection.
But soft body parts like stomachs and other fleshy bodily organs and appendages in the wild collection of creatures found at the Burgess Shale? You really have to be lucky to get samples of any of those from the distant past. About 20 percent of the 140 or so original species found in the Burgess Shale were skeletonized, and the rest were soft-bodied. Still the earth that captured these creatures clearly displayed their ghostly impressions. This incredible find is preserved in a section of the shale about the height of a man and not quite as long as a city block, and according to Gould it has “more anatomical disparity than in all the world’s seas today.”
In a burst of evolutionary creativity, all the major body plans suddenly appeared onstage. Although some scientists wonder if the original cast was so varied, Richard Leakey argues that as many as seventy actors were present, displaying the different body plans or phyla of life. But what remains today are perhaps only thirty or so such plans, the others having been cut from evolution’s cast since the Burgess Shale was formed.
The Smithsonian’s Charles Walcott was of a more conservative opinion. He originally categorized all the creatures he found in the Burgess Shale as a part of the recognized phyla or body plans of today. But in the late 1960s, Harry Blackmore Whittington, a paleontologist at Cambridge University, reopened Walcott’s excavation to take another look. As at the Capitan Reef at Guadalupe National Park, these glorious remnants of past life were entombed in the crest of a mountain, but they had once inhabited an ancient sea. The residents of this ocean community had been caught by mudslides, which preserved their bodies in ghostly detail as flattened images in thin layers of shale.
They were an odd bunch, mostly small but truly varied and exotic: Opabinia had five eyes and a long, flexible trunk tipped with a grasping spine. Amiskwia looked like a strange seal with a rattlesnake’s head. Anomalocaris had underwater wings, shrimp tails for arms, and a scary mouth with a ring of sharp teeth for cracking the bodies of scorpions, spiders, and shrimp. Wiwaxia had a series of spines projected in two rows along its back, looking like a bear trap ready to be sprung. And last but not least, there was Pikaia, a worm an inch and a half long—man’s early ancestor.
The Burgess is our best example of the Cambrian explosion, a period during which life jumped from a simple and not too varied existence to the ancestors of the fullest complexity of nature we’ve seen on the planet. The Cambrian explosion perplexed Charles Darwin because it offered another refutation of his conception of evolution as a slow and steady progression. Here, life quite suddenly made an enormous leap.
The development of vision during the Cambrian was one of the great inventions of nature that may have helped ignite the Cambrian explosion, transforming the behavior of all living creatures. It put prey at a whole new level of desperation. Predators could better spot and chase prey. This led to the evolution of shells and the tough exterior skeletons of crustaceans, giving prey a chance at survival. It also provided a much greater likelihood that these creatures would appear as fossils in the rock record because those tough exteriors survived time.
Movement was another one of nature’s great inventions, but appears to have shifted gears in importance after the Permian mass extinction 250 million years ago. Life in the Permian oceans was largely anchored to the bottom. Lampshells, sea lilies, and shellfish filtered food from the water, a meager though lazy way to make a living. But after the Permian extinction, things that moved dominated the animal kingdom. This new skill buffered life from sudden change in the environment, allowing it to develop.
But another important aspect was that movement led to complexity. Nature was more diversified after the Permian. Rather than a handful of species that dominated the landscape, with the rest left to eke out a living, multiple species began to abound and thrive in conjunction with each other. The number of species living together increased dramatically in the fossil record and laid the foundation for the world we live in today.
AFRICAN SOJOURN
Animal life has grown quite larger and more complex since the Cambrian explosion. To see evolution at work I visited the Ngorongoro Conservation Area in Tanzania, Africa. Man has devastated large animal populations in most places on earth, but in Africa these two evolved simultaneously and wildlife adapted to keep their distance. Nowhere on earth except in Africa are there so many large animals, though even here they are subject to human voracity. Evolution, however, is helping some animals adapt to man by shedding their tusks and horns.
To view this firsthand I traveled one summer day with Joseph Masoy, a smiling, husky Tanzanian, who commanded his Toyota Land Cruiser over bumpy African roads to get into Ngorongoro Crater, the relic of an ancient volcano that was once filled with lava but is now filled with African wildlife. Seated with me i
n the truck were Nicholas Toth, Kathy Schick, and James Brophy, all professors at Indiana University, who were traveling to Olduvai Gorge. The car contained several weeks’ worth of gear, supplies, and personal belongings, as well as a pop-up roof that allowed us to view and take pictures of the wildlife along the way without getting eaten.
We gradually approached the green jungle that shrouded the Crater Highlands of the Eastern Rift Valley, as the sun boiled up the midday tropical clouds into the sky. By early afternoon we crested the rim of Ngorongoro Crater and descended into the ancient cauldron. Ngorongoro Crater became a UNESCO World Heritage Site in 1979. At our first look into the crater, it seemed vacant: some little specks down there—rocks perhaps—but not much wildlife.
As we wound down the inside wall of the cauldron, these specks grew more and more spectacular. The first group we came upon turned out to be a herd of Cape buffalo. For the most part the animals ignored the tourist vehicles. They moved about in small groups and herds, feasting on the savanna grasses that covered the crater’s floor. One buffalo stood and stared at our truck. It appeared mean and perturbed. Masoy claimed that buffalo are some of the most dangerous of Africa’s wildlife, partly because there are so many of them and partly because people don’t take them seriously. I could only count them in batches, each containing perhaps fifty animals. There are at least twenty other batches within our field of view, perhaps one thousand animals in all.
We spot a pair of rhinos. They keep their distance, maybe two hundred yards away. That afternoon we spot about fifteen hundred zebras, two thousand wildebeest, one thousand buffalo, several bustards (a large terrestrial bird), black-crowned cranes, impalas, six hyenas, about eight jackals, one African lion, one cheetah, and eight giraffes.
But the surprise of the day came when we spotted three elephants in Ngorongoro Crater walking through a crowd of several hundred zebra. One elephant was tuskless; another seemed to have broken one of its tusks. None had a glorious pair of ivory as in your typical African photo. This was evolution in action. The tusks of the big elephants are a gold mine and too dangerous for the animals to carry.
Despite the government threat to shoot poachers on sight, poachers keep trying. Similar to other parts of the world, Africa is losing its animals. Neither strict national laws nor international support nor tourist income completely protects these majestic animals from illegal hunters. Congolese authorities recently accused the Ugandan military of killing twenty-two elephants from a helicopter and then carting away more than a million dollars’ worth of ivory.
In the 1970s, 10 to 20 percent of all the elephants in the wild were killed. At that rate, extinction could have come quickly, but international pressure and evolution have given the elephants a reprieve. Poaching put evolutionary pressure on animals with tusks, and tusks on elephants began to disappear rapidly. Ownership of ivory tusks was too expensive.
Selection has affected both male and female elephants. A Prince-ton ecologist, Andrew Dobson, traced the evolution of tusklessness in females at five African wildlife preserves. In one park where elephants were relatively safe, the incidence of tusklessness in females was small, a few percent. But in another park where they had been heavily poached, it was a different story. Females aged five through ten were about 10 percent tuskless. But females aged thirty to thirty-five were about 50 percent tuskless.
Researchers have noted similar results for males. That nature would allow male elephants to give up their tusks is phenomenal. Males use their tusks to battle each other for access to the females. A male without tusks is like a knight without a lance; yet, due to the state of game hunting, tuskless males have a better chance of surviving. Thus nature now selects for tuskless males as well as females.
Adapting to man is currently wildlife’s greatest evolutionary challenge. The animals in Ngorongoro Crater, including the human ones in the safari vehicles, are all descendants of our common ancestor Pikaia. Yet we are presently locked in mortal combat.
The diversity of life is present in Africa’s game preserves, but one wonders how it began and how long it can continue.
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THE GROUND BELOW THE THEORIES
MOST SCIENTISTS AGREE that the slow, lateral movement of the continents, their joining and separating on the surface of the planet, has strongly influenced the broad diversity of plants and animals that exist on earth today. During the Permian period all the continents joined together in one enormous landmass, a supercontinent, but after that extinction event, the supercontinent Pangaea began to split apart like a broken dinner plate with its pieces scattered across the oceans. This separation of the landmasses led to a corresponding separation of species of plants and animals. Newly separated species no longer exchanged genes with one another and over time isolated populations evolved away from each other and became separate species.
The splendor of this evolutionary tale was on display when Darwin and the crew of HMS Beagle rowed up to the island of San Cristóbal, a black mound of volcanic rock in the middle of the Pacific Ocean, during their celebrated visit to the Galápagos Islands that began on September 17, 1835. At a distance the island looked desolate, but upon landing Darwin found it covered with plants that bore leaves, flowers, and seed-bearing fruits. He had been on a four-year expedition to South America and was now heading across the Pacific on the long way back home to England. The crew of the boat hoped to catch a tortoise for some meat to make a tasty soup, but there were no tortoises on San Cristóbal.
He did find numerous birds, which were unfazed by human presence as they looked for seeds in the bushes; they’d had no experience with humans. One of the crew even caught a bird with his hat. Darwin picked up an iguana and threw the animal into the water over and over, but each time it swam straight back to him. He yanked on the tail of another that was digging a burrow, and it turned and looked at him as if to say, “What made you pull my tail?”
The Beagle docked in the Galápagos Islands for five weeks, during which Darwin accumulated plants and animals, focusing on the many birds. He thought he was collecting blackbirds, wrens, and warblers, but when he got them back to London, an ornithologist told him that though the birds looked different they were all finches. Plus Darwin had stored birds in bags by type and hadn’t separated much of his collection by island, which he later found was important. He’d assumed they were the same species he’d seen on mainland South America.
He did notice that the mockingbirds he’d taken on the second island seemed different from the ones on the first, so he started labeling them. When the vice governor of the islands told Darwin that he could distinguish the tortoises on one island from the tortoises on another, Darwin ignored him at first. Darwin did not imagine that these animals could have originated from a few animals blown across the Pacific and that they had diversified into different species on different islands within clear sight of one another. Darwin held, as did many scientists at that time, that these animals were all the same. Differences in color and form were indicative of different varieties, not separate species.
The definition of a species, according to Ernst Mayr, a German-born American biologist, is “groups of interbreeding natural populations reproductively isolated from other such groups.” This definition didn’t seem to fit the samples of wildlife Darwin had collected. These islands were in sight of one another. Surely separate species could not form on places so close. But they had indeed.
When Darwin returned to England, he gave all his bird skins and other trophies to the Zoological Society of London, and the ornithologist John Gould took a fresh look at them. At the next meeting of the society, Gould professed his excitement over Darwin’s findings of a new group of “ground finches.” The Daily Herald the next day reported on the meeting, noting the fourteen species of ground finches, “of which eleven were new forms none being previously known in this country.” This finding heralded an important moment in the evolution of Darwin’s On the Origin of Species, though it would be twenty-three more years bef
ore the book was published.
The fossils Darwin collected in South America were unique as well. Among them were a giant llama, a giant armadillo, and a rodent as big as a rhinoceros. Wherever one followed the trail of life, across the land or back through time, “species gradually become modified,” wrote Darwin. He was beginning to realize how new species might evolve, but he had no idea at the time what a large role continental drift had played in the process.
On the voyage of HMS Beagle, Darwin brought Principles of Geology by Charles Lyell along for reading. Though his Cambridge professors had warned him to take the book with a grain of salt, he enthusiastically accepted Lyell’s view of the earth changing restlessly beneath man. Darwin had witnessed this change in his journeys through South America. Still, both thought the movement of the continents was upward and downward, and that nothing moved laterally.
Darwin had no idea yet how important both the vertical and horizontal movement of the continents on the surface of the earth was to evolution.
GEOLOGY LED THE WAY
The mid-1800s were a time of upheaval in biological as well as geological thought. The British Empire was in full bloom and the most famous of the early geological surveys date from this era. The Industrial Revolution had arrived earlier with an insatiable hunger for iron, coal, oil, and other deposits, and thus geologists became the celebrities of the day. They earned their keep by uncovering industrial resources, and in accordance with the spirit of discovery that ruled then, these geologists weren’t afraid to address more theoretical issues, like how these resources came to be.
Brothers William and Henry Blanford, members of the Royal School of Mines in London, were offered posts with India’s newly hatched geological survey and were sent to investigate the Talcher Coalfield in the state of Orissa in that country. The Blanfords started digging and in 1856 found that below this enormous bed of coal was yet another formation of large boulders embedded in fine mudstone, and there was telltale evidence of a glacier. The boulders all had the markings of glacial scour—the abrasions, scratches, and polish of glacial ice against rock. Furthermore, some of the boulders had been moved large distances, another telltale sign of glacial action.
The Next Species: The Future of Evolution in the Aftermath of Man Page 4