Typically, we conceive of life on Earth as beginning around 540 million years ago, with the rise of oxygen, land plants, certain marine invertebrates, and eventually the dinosaurs—life as we relate to it. But an unusual new chart by researchers at Harvard and at Howard Hughes Medical Institute illustrates exactly how the story extends much farther back in time.
Unlike most narratives of Earth's life, this one illustrates connections between biological and geological processes: plate tectonics, chemical cycles, the greenhouse effect, early photosynthesis, and the oxygenation of the planet—a process that provides a theme to Earth's saga. The early Earth had elements like carbon and nitrogen needed to create organic molecules, and, as researchers have hypothesized, geothermal vents that could have driven chemical reactions by creating drastic fluctuations of temperature in the water. But as life evolved, photosynthetic bacteria filled the atmosphere with oxygen. The gas began to appear about 2.4 billion years ago; it wasn't until roughly 550 million years ago, during the most recent era, when large amounts of the gas began giving rise to more developed kinds of life.Climate change is also a major character in Earth's tale: while some of the mass extinctions that have punctuated the the Phanerozoic Eon (the "age of visible animals" that began 541 million years ago) are thought to have been caused by large meteorites, volcanos are also a culprit, for causing a rapid increase in atmospheric carbon-dioxide levels. "The one process we know that increases CO2 at rates comparable to those of massive volcanism is human industrial activity," Andrew Knoll, a professor of natural history who worked on the chart, told Harvard Magazine, "thus linking our biological past and future.”Understanding the dynamic relationship between geology and biology is also helping science to dig deeper into the lives of distant Earth-like planets, and better understand what makes a planet potentially habitable to life, both ours and other as-yet-unknown kinds, ones that may, for instance, be based on silicon rather than carbon.The search for new planets has been heating up recently, and just in time too: while biologists in England have estimated that while the Earth has at most 3.25 billion years left, history indicates that we're not likely to make it that long. (Al Gore recently told my colleague Brian Merchant that if we're not careful our time could be up in the next hundred years.) After the human species eradicates itself through a rise in temperature or some other kind of extinction event, the story of life on Earth would likely return to where it started. "Near the end," said researcher Andrew Rushby, a researcher at the University of East Anglia, " only microbes in niche environments would be able to endure the heat.”