杏吧原创

MOST accounts of life on Earth begin little more than half a billion years ago. That is when an evolutionary burst of creativity produced the ancestors of almost all animals and plants alive today. Following this 鈥淐ambrian explosion鈥�, life鈥檚 story is one of fish, amphibians, insects, land plants, the rise and fall of the dinosaurs, and ultimately the emergence of humans. It is an epic tale 鈥� but it spans just one-eighth of life鈥檚 history.

The problem is that although animals and plants have left abundant fossils, Precambrian rocks contain almost no traces of earlier life. This vexed Charles Darwin, who wrote in On the Origin of Species: 鈥淭o the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.鈥� Since then, a few fossilised remains have been found, but these are mostly microscopic blobs, reluctant to give up their secrets. Yet, in recent years, ingenious researchers have found new ways to lift the lid on life鈥檚 black box.

This is the story of the first 3.5 billion years. It is a tale dominated by single-celled organisms, but it is also one of cataclysmic change. It encompasses the birth of the continents, the greatest act of chemical pollution ever committed, and a freak evolutionary event that may never have happened anywhere else in the universe. It is an epic journey, so buckle up.

In the beginning was Earth. from rocks and dust and, soon afterwards, was smacked in the face by another baby planet (). The impact melted Earth鈥檚 surface and threw chunks of matter into orbit, . Where one vital ingredient for life, Earth鈥檚 water, came from is a long-running debate. It may have been locked up in the rocks that formed the planet, been brought in later by comet, or perhaps it came from interstellar space and is older than the sun itself.

鈥淟ife might have been easier to get going than was once thought鈥�

Exactly when life got started isn鈥檛 clear. 鈥淭he oldest uncontroversial evidence we have is the Pilbara Strelley Pool stromatolites,鈥� says Greg Fournier of the Massachusetts Institute of Technology. Found in Western Australia, these preserved . They cannot be the first life, though, because the cells have complex membrane structures and link together into chains. They look like 鈥渁 complex, diversified bacterial ecosystem鈥�, says Fournier.

Claims of older fossils are regularly touted. And the appear to date from 4.1 billion years ago. But this evidence is controversial, partly because Earth is thought to have been battered by meteorites between 4 and 3.8 billion years ago, rendering it uninhabitable. However, over the past few years, this 鈥渓ate heavy bombardment鈥� was reassessed. Simulations suggested that rather than happening in one short, sustained blast, large meteorites kept hitting at a low rate until about 3 billion years ago. These impacts would have caused problems for life, but couldn鈥檛 have obliterated it.

That helps explain why there was little pushback this August, when a new calculation put the origins of life firmly before 3.8 billion years ago. Davide Pisani at the University of Bristol, UK, and his colleagues compiled genetic data from 102 living species and built a family tree showing how they were related. Then they took some firm dates from the fossil record and used these to work out how fast evolution has been happening, allowing them to estimate . One of these was the origin of the last universal common ancestor (LUCA), which they calculated lived at least 3.9 billion years ago. The very first organisms would have arisen even earlier 鈥� remarkably near the start of Earth鈥檚 history 鈥� hinting that life might have been easier to get going than once thought.

We already have some idea of what LUCA was like. Several teams have used genetic methods similar to Pisani鈥檚 to figure out which genes it had and which evolved later. by William Martin at the University of D眉sseldorf, Germany, and his team. They identified 355 genes that LUCA probably possessed. These indicate that it lived somewhere hot and used carbon dioxide to make acetate, which served as its food. It did not, however, possess the mechanisms for making amino acids 鈥� the building blocks of proteins 鈥� so probably acquired them from the environment. LUCA, it seems, was a complex organism, but still missing some key abilities.

Over the next billion years, life diversified. According to Pisani鈥檚 study, the first big split took place at least 3.4 billion years ago. It resulted in two groups of microorganisms, bacteria and archaea. Although they look almost indistinguishable under the microscope, they have distinct biochemistry. In particular, many archaea have evolved to live under extreme conditions such as high temperatures. They also nourish themselves in unusual ways, including a unique method called methanogenesis. So-called methanogens harness either carbon dioxide or acetic acid to obtain energy, releasing methane as a waste. Their evolution played a key role in the history of life on Earth.

鈥淚n the beginning, oxygen was a deadly form of pollution鈥�

Fournier and his student Jo Wolfe have now worked out when methanogenesis originated. Using an approach similar to Pisani鈥檚, they calculated that it emerged in some archaea a whopping 3.5 billion years ago, or more. comes from chemicals in rocks indicating there was methane in the air by then. It fits with some global changes that were happening too. 鈥淎tmospheric models suggested that the Earth would have had liquid water at that time, but we also know that the sun wasn鈥檛 as bright as it is now,鈥� says Wolfe. 鈥淭he sun alone couldn鈥檛 have been warming ice and melting it.鈥� But methane, a powerful greenhouse gas, would have lent a hand. So, it looks like archaea were the first organisms to affect Earth鈥檚 climate. And, by keeping the planet warm, they propped up the rest of the primordial ecosystem too.

Methanogenesis was revolutionary, but it is an inefficient form of nutrition. Research published in April reveals how . First it relied on chemical reactions that yielded little energy, including methanogenesis. Later, some archaea evolved the ability to use sulphur, which is a better source of energy. Then others began to use something even more effective: oxygen. But that was only possible once there was oxygen in the air. So where did it come from?

For the first 2 billion years of Earth鈥檚 history, there was no free oxygen. Instead, it was locked up in minerals and other chemicals because it readily reacts with different elements. All that changed 2.4 billion years ago with the 鈥済reat oxygenation event鈥�. The driving force was the rise of cyanobacteria, which harness energy from sunlight. In fact, other bacteria had evolved the capacity for photosynthesis starting about 3.5 billion years ago. But cyanobacteria were the first to use the sun鈥檚 energy to fuse carbon dioxide and water into sugar, releasing oxygen as waste. In doing so, they became the second organisms to have a global impact.

The air now held oxygen. Levels were far lower than they are today, and stayed low for more than a billion years. Nevertheless, oxygen helped trigger one of the most dramatic events in Earth鈥檚 history: an ice age so severe that all (or almost all) the planet froze over. This first 鈥渟nowball Earth鈥� began shortly after oxygen appeared in the air, and lasted until 2.1 billion years ago. by produced by methanogens, reducing the chemical鈥檚 greenhouse effect. A second cause was the emergence of the first large continents (see 鈥�A short history of rocks鈥�). Being paler than the seas, they reflected more of the sun鈥檚 heat back into space. Finally, chemical reactions associated with the weathering of newly formed mountain ranges , carbon dioxide, from the air.

We tend to think of oxygen as being essential for life, but in the beginning it was a deadly form of pollution, and its accumulation in the air probably caused the first mass extinction. Today, oxygen is toxic to 鈥渁naerobes鈥� 鈥� organisms that live in oxygen-free places 鈥� so it was presumably lethal to many ancient microbes, says Fournier. The ice on snowball Earth would also have destroyed many habitats. However, the microbial fossil record is too sparse to show this. 鈥淭here is no way to empirically detect a microbial mass extinction,鈥� he says.

Not so boring billion

Following this chilling upheaval came a period of calm 鈥� or so we thought. Conventional wisdom has it that, with the climate stable and oxygen levels low, evolution stalled. Life鈥檚 history was considered so dull that the period between 1.8 billion and 800 million years ago has been dubbed the 鈥渂oring billion鈥�. In fact, it was nothing of the sort. This is when life made two of its biggest advances. And, according to recent research, these evolutionary innovations may have been the result of organisms rising to the challenge of .

Whatever the reason, the first big leap was the emergence of a new kind of organism, called a eukaryote. Pisani鈥檚 study estimates that this happened between 1.8 and 1.2 billion years ago, deep in the boring billion. Eukaryotic cells are larger and more intricate than anything that came before. They have complex internal membranes, their DNA is stored in a chamber called a nucleus, and they contain tiny, sausage-shaped objects called mitochondria that supply them with energy. Only eukaryotic cells can form complex, multicellular organisms like plants, insects and humans 鈥� bacterial and archaeal cells cannot link up in such fiddly ways 鈥� so the origin of eukaryotes is one of the most important events in the history of life.

Eukaryotes arose not through competition, which drives so much of evolution, but by cooperation. At some point, an archaean must have swallowed a bacterium and the two organisms managed to live as one, with the bacterium becoming the first mitochondrion. Quite how this happened is still debated. We may know more soon by studying a recently discovered group of organisms called the Asgard archaea, which are the . However, the creation of complex life seems to have occurred only once, suggesting it was a freak event. If so, that has cosmic implications. It means that although other planets could be home to simple cells, these might thrive for aeons without complex life ever arising.

With the evolution of eukaryotes, life on Earth could take another big leap and become multicellular. So, when did that happen? Until the 1950s, the oldest known multicellular organisms were from the Cambrian period, which started 541 million years ago. Geologist Reg Sprigg found fossils that later turned out to be Precambrian at Ediacara in the Flinders Ranges of South Australia in 1946 but received little attention. , which was the first recognised Ediacaran, a weird, multicellular organism that lived between 635 and 542 million years ago. The nature of this group has long been a mystery. A new study suggests that at least one of them, called Dickinsonia, was an animal 鈥� the earliest known, at half a billion years old. Ediacarans weren鈥檛, however, the first multicellular organisms.

鈥淧erhaps evolution was slowed when Earth turned into a giant snowball鈥�

In 2016, a team led by Shixing Zhu of the China Geological Survey described that were up to 30 centimetres in length. They look like primitive seaweed and were found in Chinese rocks dated at 1.56 billion years old. Fragments of preserved cellular material revealed the cells to be organised in tightly packed sheets, not in loose colonies, strongly suggesting they were true multicellular organisms. This year, found that, until about 1.57 billion years ago, the ocean where those organisms lived was low in oxygen, then levels rose, suggesting that it was the extra oxygen that enabled them to grow larger and more complex.

People used to wonder why multicellular organisms took so long to evolve. Now we have a new puzzle: why it took them a billion years to develop into the more complex forms of the Ediacarans. Perhaps oxygen levels had to rise still further. Or perhaps evolution was slowed in its tracks by a second snowball Earth period that occurred between 720 and 635 million years ago. However, of species being wiped out at that time. Indeed, it has been argued that some organisms thrived on Earth鈥檚 icy surface.

So, mysteries still remain. Life鈥檚 Dark Ages have yet to be fully illuminated. But we know far more than we did just a few years ago. And it is clear that life as we know it today would not exist without all the innovations that happened in the first 3.5 billion years of evolution.