When Life Became Large

‘For another billion years or so, eukaryotes remained, like bacteria, microscopic. But around 1.8 billion years ago the first multicellular fossils appear, in the form of mysterious coils measuring about 2 centimeters long. The oldest recognizable multicellular organisms are red algae dating back 1.2 billion years. Our own multicellular lineage, the animals, don’t leave fossils until about 575 million years ago. “Animal” is a generous description for the creatures that left these marks. There are disks with three‑pronged ridges on them, like coins from a vanished empire. There are fronds with rows of slits that must have looked like underwater Venetian blinds. There are ribbed impressions on the old seafloors shaped like gigantic thumbprints.

These creatures are known as the Ediacaran fauna (named after the Ediacara Hills in Australia where they have been found in abundance). In the past paleontologists have tried to classify them as plants, lichens, or even some failed experiment in multicellular life. These days, many experts think that at least some of them are early relatives of the major groups of living animals (known as phyla). Some of the fossils might be related to jellyfish. The giant thumbprint may actually be kin to the annelids, a group that includes earthworms and leeches. The fronds may be sea pens, which live today on coral reefs. But there are still many Ediacarans that remain up for grabs. No one has taken on the coins yet.

Tucked away among these fossils of Ediacarans are signs of the future of the animal kingdom. In rocks as old as 550 million years there are traces of burrows and tracks that could only have been made by muscular animals. They point to the existence of complex animals that could dig and crawl, as opposed to the rooted Ediacarans and the drifting jellyfish. These phantom animals may have already evolved many of the hallmarks of complex animals, such as a muscle wall and a gut. Primitive animals such as jellyfish lack these structures, while animals such as insects, flatworms, starfish, and people have them. The difference comes down to how their embryos form. Jellyfish are composed of two body layers (biologists call them diploblasts). Other animals have three layers: an ectoderm, which ultimately produces skin and nerves; mesoderm, which forms muscle, bone, and many internal organs; and endoderm, which builds the gut. These three‑layered animals are triploblasts. Triploblasts presumably made the burrows and tracks 550 million years ago, but paleontologists have yet to find their fossils.

In 1998 paleontologists in search of these early triploblasts made a promising discovery: Precambrian embryos. A team of American and Chinese researchers found a cache of microscopic fossils dating back 570 million years. Some are single‑celled fertilized eggs. Some are at the next stage of division, a two‑cell ball. Some have four cells, eight cells, sixteen, and so on. The paleontologists have no idea what exactly these embryos would have grown up to be, but judging from their size and the pattern of their division, the best candidates would be triploblasts.

By 530 million years ago, in the early Cambrian period, the Ediacarans had declined and disappeared. At the same time the fossil record of triploblasts exploded. Among the fossils of this time you can find the earliest clear‑cut relatives of many of the major groups of living animals. Our own phylum, the chordates, is represented by fossils of creatures that looked like lampreys and hagfish–Owen’s vertebrate archetype made flesh.

Other phyla made a more extravagant entrance. A relative of today’s mollusks looked like a pincushion studded with arrowheads. Living lamp shells were foreshadowed by Halkieria, which looked like an armored slug. Opabinia had five eyes sprouting like mushrooms from its head, and it stirred up the seafloor with a clawed nozzle that it could also use to grab prey and stuff into its mouth. Opabinia now appears to be an early relative of living arthropods. Other phyla that today live in humble obscurity–velvet worms or peanut worms, for instance–gloried during the Cambrian explosion in a diversity that they would never again enjoy.

Darwin’s worries over the Cambrian turn out to be unfounded. Now that scientists can read isotopic clocks and recognize molecular fossils, they have shown that the world did indeed swarm with life for billions of years before the Cambrian, as Darwin proposed. The Precambrian, far from some mysterious prologue to evolution, actually takes up 85 percent of the history of life. And paleontologists now have a marvelous collection of Precambrian fossils, including bacteria, protozoa, algae, Ediacarans, burrow makers, and animal embryos. But even with a much smoother fossil record, the Cambrian period is clearly the most remarkable episode in animal evolution. No matter how long animals were already lurking in the oceans, their diversification accelerated 535 million years ago in a tremendous explosion. Thanks to precise uranium‑lead dating, scientists have determined that the Cambrian explosion took only 10 million years.

The Cambrian explosion took place completely underwater. As these new animals came into existence, the continents were bare, except for bacterial crusts. But it was not long, geologically speaking, before multicellular life spread onshore. First came plants. Around 500 million years ago green algae gradually evolved waterproof coats that allowed them to survive exposed to air for longer and longer periods of time. The first land plants probably looked like today’s moss and liverworts, forming a low, soggy carpet along the banks of rivers and coastlines. By 450 million years ago centipedes and other invertebrates were beginning to explore this new ecosystem. New species of plants evolved that could hold themselves upright, and by 360 million years ago trees were growing 60 feet high. Out of the coastal swamps would sometimes slither our ancestors–the first vertebrates that could walk on land.

In the history of life, the move to land is a brief coda. Nine‑tenths of our evolution took place completely underwater. But from our point of view, the last few hundred million years on land are the most interesting period of all. Fossils of the earliest land vertebrates show that they had branched into two lineages by 320 million years ago. One branch, the amphibians, produced some lumbering giants early on, but today is represented only by frogs, salamanders, and other small creatures. They generally need to stay moist and lay soft eggs that can dry out easily. The other branch, the amniotes, evolved a strong water‑tight eggshell. From among these amniotes the dinosaurs emerged around 250 million years ago; they evolved into the dominant land animals and stayed that way until 65 million years ago, when most dinosaur branches became extinct (the only survivors were birds, which are just feathered flying dinosaurs). Although the first mammals appeared alongside the first dinosaurs, they didn’t dominate the land until their reptile counterparts disappeared. Our own primate lineage probably emerged around then, but it wasn’t until 600,000 years ago that the oldest fossils of Homo sapiens were buried in the earth. All humans alive today can trace their heritage back to a common ancestor that lived only around 150,000 years ago.

While these few pages can’t do full justice to the majestic depths of life’s history, one thing is clear: our own time in this universe is almost inconceivably brief. No longer can human history match the scale of natural history. If the 4 billion years that life has been on Earth were a summer day, the past 200,000 years which saw the rise of anatomically modern humans, the origin of complex language, of art, religion, and trade, the dawn of agriculture, of cities, and all of written history–would fit into the flash of a firefly just before sundown.

In the end, Darwin got the luxury of time he craved. But the fossil record, though it documents the pattern of life’s evolution, does not reveal the details of just how it evolved. And on this point, Darwin also never managed to close his case during his lifetime, because he never understood how heredity works.

While geologists and paleontologists were charting the history of life, other scientists in the twentieth century solved heredity’s mystery and linked it to natural selection. The connection lies among molecules and atoms, as did the proof of life’s antiquity. But the atoms of heredity are not embedded in rocks for billions of years. They are lodged in the core of our own cells.

Four

Witnessing Change