The Ocean of Life
Page 3
Why did it take so long to ventilate the deep? Oxygen levels in the atmosphere were still very low, no more than about 1 percent of today’s. We would have suffocated in no time. Oxygen is transported to the deep sea when surface waters plunge down, but it is used up to break down organic matter—dead microbes, in other words—that sinks from the surface. It would not take much sinking organic matter to strip oxygen from the deep faster than it could be replaced. Still, it is hard to explain how deep water could remain oxygen-free for such an immense stretch of time after the rise of oxygen-producing photosynthesis. Early oxygen producers lived in shallow, sunlit waters. Just like life today, they needed nutrients to fuel their growth as well as sunshine. Since most nutrients sink with the dead bodies of microbes, they have to be recycled by upward mixing of deep water. An intriguing new idea is that anoxic waters rich in hydrogen sulfide reached into the lower sunlit layer, where they encountered photosynthesizers that used up hydrogen sulfide but didn’t make oxygen, and intercepted most of the nutrients before they could reach the oxygen-producing cells closer to the surface.28 There is another twist in the nutrient tale. Very early on life incorporated the trace metals iron and molybdenum into enzymes that fix nitrogen, thus securing one of life’s essential nutrients. Sulfidic seas would have little of either of these metals in solution, because both would have reacted with hydrogen sulfide to produce insoluble compounds that were deposited at the seabed. So it was that quirks of oceanography and limitation of nutrient supply appear to have held back planetary oxygenation for what seems like eternity.
There are still places today where sulfur-based photosynthesis continues. The Black Sea is an almost completely enclosed basin of water that is capped by a warm, sunlit surface layer that is less dense than the cool water underneath. Little of the oxygen in this warm surface layer mixes downward, so the Black Sea has not had any oxygen for thousands of years below about five hundred feet. If you were to somehow bring a sample to the surface it would stink of rotten eggs, just as the sulfidic ancient oceans would have. As this anoxic water nears the surface, green and purple sulfur bacteria use sunlight to produce food and energy, just as their predecessors did eons ago.
It is hard to know if all the world’s oceans were once sulfidic like today’s Black Sea. Most of the rocks that could tell us were long ago recycled back into the mantle, so there are few places to look for clues. What we do know is that eight hundred million to nine hundred million years ago conditions began to change. What paleontologists refer to as the “boring billion years” ended. Ocean chemistry shifted toward the composition of modern oceans during this period. A phase of intense mountain building around this time may have enhanced the delivery of trace nutrients to surface waters, enabling oxygen producers to flourish. With the amount of oxygen in the atmosphere rising, the sea became ventilated beyond the lower limit of the sunlit layers, ending the hegemony of the sulfur bacteria and freeing more nutrients for oxygen producers. This positive feedback led gradually to the irreversible oxygenation of our atmosphere. It set the scene for life’s next great innovation: multicellularity.
When Charles Darwin wrote On the Origin of Species, there were fossils in abundance all the way back to the onset of the great Cambrian explosion, when life-forms with hard body parts which readily fossilized first came into being. Before that, nothing: The rocks appeared blank. It was as if some creator had conjured forth a menagerie of primitive life that would later diversify into all subsequent beings. It has taken a hundred and fifty years of intensive search to find the enigmatic shadows of earlier life, much of it microbial, but it is there. Darwin would have loved to know what we know today.
Oxygenation of the Earth’s atmosphere was a key event in the history of life. A metabolism based on oxygen respiration produces sixteen times more energy than equivalent anoxic pathways. When free oxygen became available it created tremendous opportunities, which were soon realized. The first oxygen-using enzymes have been traced by following the evolution of proteins backward in time, using the latest genetic sequencing libraries and powerful computer techniques. Although there are precious few pre-Cambrian fossils, in a sense every one of us contains a fossil library of genes that goes all the way back to the Archean eon, billions of years ago. The history of changing protein shapes gives us a molecular clock from which we can read the time when new innovations first emerged. The first hesitant steps toward the use of free oxygen seem to have been taken 2.9 billion years ago, 400 million years before the first clear evidence of free oxygen appears in the rocks.29 Proteins involved in oxygen metabolism multiply fast through the Great Oxidation Event. Then there is a second wave of evolutionary innovation in oxygen use that begins about 1.2 billion years ago and continues all the way to the Cambrian explosion of life. Atmospheric oxygen had increased to about 12 percent of today’s level by the onset of the Cambrian period.
Oxygen is produced by photosynthesis when organic matter is created, and consumed both by respiration and by the decay of that organic matter. If the two sides of the equation were exactly equal, oxygen would not build up in the air. But some organic matter is buried at the bottom of the sea or in swamps and lake beds. During late pre-Cambrian times, when oxygen levels in the atmosphere rose, the rate of organic carbon burial must have increased. Free oxygen is also produced when sulfate combines with iron to produce iron sulfide (pyrite, or fool’s gold). So increased rates of pyrite burial would also oxygenate the world. We don’t know exactly what caused burial rates to increase, but the oxygenation of the oceans is believed to have played a key role.
Whatever the cause, there are many who think that rising oxygen levels paved the way for life’s explosion. The two go hand in hand. Large-bodied aerobic animals need enough oxygen to ventilate their tissues. Their virtual absence in the previous three billion–plus years of life might just be because their size was impossible until oxygen levels rose high enough to sustain such bulk.
The Cambrian period was a time of amazing evolutionary creativity. We can read in the rocks, within 20 million years of its onset, 542 million years ago, the appearance of virtually every major animal group alive today. If periods are characterized by their most successful creatures, this was the age of the arthropods, which are distinguished by their external skeletons and jointed legs. Insects, crabs, lobsters, millipedes, and spiders and the like are all their living descendants. But the now extinct trilobites really define the Cambrian period better than any other group. Cambrian seas were full of a bewildering mix of these low-bodied, scuttling creatures, which wore body plates and helmets that were sometimes flamboyantly ornamented, with spikes and spines and compound eyes with crystalline lenses. Trilobites made good use of this articulated armor to defend themselves from the predators that were about. One explanation for the Cambrian explosion describes it as a runaway arms race in which some animals scrambled to escape or repel predators, while others developed ever better ways to catch prey. Grazing and predation predate the Cambrian period by hundreds of millions of years, but it was then that the ability to swallow large prey was firmly established. The foundations of modern food webs were laid in Cambrian seas, a period that came to a close 488 million years ago.
Although life was diversifying, a series of environmental crises lopped branches from the spreading crown of the tree of life. Just when the atmosphere had become more breathable, anoxia returned to the oceans and sulfidic seas reappeared.30 What seems to have happened is that waters devoid of oxygen welled up onto shallow continental shelves, perhaps following rising seas, and they snuffed out many trilobites and other creatures. Paradoxically, these crises may have driven atmospheric oxygen levels up. Ocean anoxia is linked to higher surface productivity and greater organic carbon burial in Cambrian oceans. Sinking feces and dead plants and animals would descend faster out of the sunlit surface layers than the microbes of more ancient seas, so more carbon would be buried, especially in anoxic waters where breakdown is sluggish. Free oxygen would get a boost.
How does this relate to sharks, you might ask? Roots and other adaptations that extracted nutrients speeded the rate at which phosphate, a key plant nutrient, weathered the soil and washed into the sea. Marine productivity boomed, enabling longer food chains and bigger predators. At the same time, land plants massively increased organic carbon burial in swamps and, through runoff, in marine sediments. This brought oxygen to levels similar to those of the present (21 percent of the air), where they have stayed, give or take 5 percent, for most of the last 350 million years. Freed from the constraint of low oxygen, the stage was set for active predators with big oxygen demands.
Animals a yard long were already present in late pre-Cambrian seas 580 million years ago. They were probably sedentary grazers or detritus feeders. Eighty million years on, there was more diversity, and hunters emerged who were the size of a small child. Fast-forward to 400 million years ago, and the seas were filled with primitive fish, including the heavily armored placoderms. Some had become terrifying beasts by 370 million years ago, as large as buses. The predatory megafauna had arrived. Over the next 120 million years they were joined by sharks and reptiles such as ichthyosaurs, plesiosaurs, and turtles. The predatory expansion reached its zenith recently, almost yesterday in geological terms. During the Miocene epoch, which spanned 23 million to 5 million years ago, giant sharks the size of great whales patrolled the sea, and whales ate whales. There were formidable sperm whales the same length as today’s but with teeth three times as big. They have been named Livyatan melvillei after Herman Melville’s mythic white whale.32
The presence of these huge predators suggests seas more productive than our own. Indeed, the oceans and their lives have had many ups and downs over the past 500 million years. The variety has gradually increased, but not in a steady rise, and not by the simple addition of new species to the existing inventory. The vast majority of species that have ever lived on Earth have disappeared. Five mass extinctions followed the crises that wiped out most of the trilobites during the Cambrian period, the ones possibly caused by re-emergence of anoxic oceans.33 The mother of all extinctions came 251 million years ago, at the end of the Permian period, when the fossils of more than 90 percent of marine species and two thirds of terrestrial species simply stop appearing in the rocks. Before the Permian extinction seabed sediments were filled with lively burrowing animals, which churned them over and over. Afterward the sediments were nearly stilled.
What caused this cataclysm remains a source of heated argument, but most likely there was a half-million-year episode of volcanism that discharged half a million cubic miles of basalt across the surface of what today is Siberia.34 The lava flooded an area of 620,000 square miles, to depths of close to 2 miles. It passed through layers of carbonate rock and coal and released massive quantities of carbon dioxide into the air. Carbon dioxide had declined more than tenfold in the approximately 240 million years after the Cambrian period, and the loss of this greenhouse gas had driven the world into a glaciation that lasted over sixty million years.
Now, at the onset of the Triassic, the period which followed the Permian, it got hot. As carbon dioxide climbed, the world warmed, and as it warmed the poles melted. Global temperatures spiked eleven degrees Fahrenheit higher than it is today. The oceans rose and the Arctic reached a balmy sixty degrees Fahrenheit to seventy degrees Fahrenheit.35 For reasons I will return to, this temporarily slowed deep-ocean mixing, and the stagnant water warmed all the way down to the deep seabed. The warm conditions melted huge polar and undersea deposits of methane, causing runaway global warming. Land plants withered and soils washed away. The stagnant sea became anoxic in all but surface waters. Meanwhile, dissolved carbon dioxide turned the oceans more acidic, which, again for reasons I will come back to, was disastrous for chalky animals like corals, urchins, calcareous seaweeds, and sponges.36 The mass extinction at the close of the Permian period almost wiped life’s slate clean. But life rose again. Within 10 million years, the oceans had refilled with new species.
Sitting in my garden one day thinking about this book, my mind wandered back to the world’s origin four and a half billion years ago. It is hard to believe that that world is the same as our own. Our planet, hurtling through the emptiness of space, so alien yet so familiar. I imagine hitting the fast-forward button to see continents ascend, meander, coalesce, and crumble; oceans rise and fall; seas, deserts, and ice caps come and go. Within the belly of these oceans creatures outlandish and familiar, bizarre and terrifying appear and disappear. Great reefs are built, destroyed, and remade. Life flourishes, is choked off and resurrects itself again and again. And through the strangeness and inconstancy swim turtles and sharks, nautilus and jellyfish, scarcely altered, like a connecting thread that reaches back through time. A moment before the tape ends, we appear. All of us carry this thread within ourselves. We are creatures of the sea with a lineage that stretches back to sponges and beyond, all the way through single-celled microbes to life’s origin. The rest of this book tells the story of what happened to the oceans after we arrived.
CHAPTER 2
Food from the Sea
Anthropology and archeology have long been in thrall to an image of early humans as big-game hunters of the open plains. Game-hunter thinking has us evolving from tree dwellers into savannah dwellers who started to walk on two legs. Sharp wits and ingenuity were necessary to thrive in the open, where we had to fend off dangerous carnivores, and our bipedalism freed us up to hold tools and weapons. Our large brains later allowed us to develop language, which unlocked the possibility of technology and culture.
This view of human origins has a certain mythological ring to it, suggesting as it does that our plucky species succeeded in a heroic struggle against great odds.1 But the story has holes. Baboons live on savannahs and have not become brilliant bipeds, and we have a mixture of adaptations that make little sense in the absence of water. Today we carry ten times more subcutaneous fat than other primates (we’re about as fat as fin whales), which if this was true of our ancestors, wouldn’t have been particularly helpful to endurance hunters running down their prey. But it would have insulated us from water. Homo erectus had dense bones more akin to those of diving mammals like manatees than fleet-footed plains hunters. We are prone to dehydration, an uncommon trait in savannah dwellers, and have an instinctual breath-hold reaction when we plunge into water. The only other primate that is regularly bipedal today is the proboscis monkey, which wades through swamp waters on its hind legs. Could our shift to bipedalism have been an aquatic adaptation developed by wading to gather shellfish?
Marc Verhaegen, a Belgian doctor, and his colleagues have recently revived the concept of man as an “aquatic ape.”2 Verhaegen believes that Homo erectus evolved at the waterside and put the fruit-and-nut-cracking techniques developed by ancestral forest species to a new use by smashing open shellfish, turtles, and crabs.
Shellfish are easy to find and gather, and although not especially calorific (it would take 150,000 cockles to match the calorie content of a large deer), they are rich in protein and other nutrients needed for brain development. The nervous system, which evolved in the oceans half a billion years ago, was built in part from omega-3 fatty acids made by algae and plankton. These compounds were in short supply for land dwellers, whi
ch helps explain why animals like the rhino, which weigh a ton, have brains two-thirds smaller than our own, while marine mammals like dolphins have large brains. Savannah models of human evolution see us getting our brain food from scavenged or hunted brains of terrestrial mammals, but waterside sources such as shellfish, waterbirds, eggs, and turtles would have been easier and more regular fare.
The idea of man as a big-game hunter has been hard to shake off. Museums freeze the thinking of the day into dioramas that endure for decades, impressing their stories into the minds of generations. I remember well from childhood the tableaux of stocky, hairy people wrapped in skins, triumphant in their slaughter of buffalo or antelope. Usually these scenes were set in open savannah, occasionally giving a nod to our dependence on water with a lake or river painted into the distant background. As Verhaegen points out, the savannah-hunter picture of human evolution predates almost all fossil finds of early hominins.3 In his accounting, all Homo fossils discovered before or since have been associated with lakes, rivers, deltas, and coasts. The concept of the aquatic ape is widely contested in anthropology, but to me it seems more persuasive than the convoluted logic needed to explain our evolution on Africa’s dusty plains.
The earliest remains of Homo species, dating from 2.5 to 2 million years ago, are from inland water bodies such as the Gona floodplain in Ethiopia and freshwater springs near Lake Olduvai in Kenya. It was later on that we reached the coast. Around 2.8 million years ago, Africa, like the rest of the world, underwent major climatic upheaval, cycling between prolonged phases in which much of the continent was arid and inhospitable and wetter phases with more varied climate. Wet phases greened the Sahara desert, perhaps opening up routes out of Africa, and arid phases pushed them into the small pockets of land that remained hospitable. One such place was southern Africa, where humans first developed their predilection for seafood. Our genes point that way, tracing our ancestry all the way back to Angola and Namibia.4