Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
Leaving aside the remote possibility that life arrived on Earth on a meteorite from somewhere else, we have to assume that it emerged from whatever physical and chemical conditions existed in the planet鈥檚 youth. Working out what these conditions were is problematical, mainly because the Earth we live on today retains almost no trace from that time.
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To date, the earliest evidence for life comes from sedimentary rocks that are 3.8 billion years old. Discovered in the 1990s in west Greenland, these rocks have an unusually low proportion of the heavy isotope of carbon. This is thought to be a sign of micro-organisms at work, because the lighter isotope passes more easily through cell walls and so accumulates wherever microbes have been.
These rocks were laid down at a time when the planet was recovering from the impact that formed the moon (see 鈥淲hat happened during Earth鈥檚 dark ages鈥�). Primordial oceans and continents were forming, but the process was interrupted every now and again by a large asteroid striking the planet and boiling the oceans. Darwin envisaged life emerging in a 鈥渨arm little pond鈥�; in fact, it was almost certainly a hot, briny cauldron.
鈥淟ife almost certainly emerged in a hot briny cauldron鈥�
This is a radically different environment from the one we live in, but perhaps that is to be expected. There are no recorded instances of an 鈥渙rigin-of-life鈥� event on modern Earth, so perhaps the right conditions no longer exist. Or perhaps it is happening on such tiny scales that we have not noticed.
Analogous conditions to early Earth do still exist. They can be found surrounding hydrothermal vents on the sea floor, where geothermal activity pumps geysers of scalding water into the ocean. These areas support vast collections of micro-organisms, many with startlingly primitive metabolisms and none of which rely on sunlight for energy. Whether hydrothermal vents were life鈥檚 point of origin or simply an early haven is unknown, however.
Another difficulty is working out exactly what happened to bring lifeless chemicals together to form living organisms. Here we are faced with a chicken-and-egg situation: for DNA to do its thing it needs proteins, yet the blueprints for those proteins are provided by the DNA. So which came first? The most likely answer is now thought to be that they evolved at the same time through a network of reactions between simpler chemicals. This makes it doubly difficult to work out when early organisms crossed from chemicals into life.
Geologists are turning to Mars for answers. There are no plate tectonics there to destroy the evidence, and sedimentary rocks can be found that date back to the time of life鈥檚 origin on Earth. The hope is that, unlike their counterparts on Earth, these rocks preserve some record of chemistry before life emerged. It鈥檚 a long shot, but they might even record an origin-of-life event that gave rise to life forms that may yet be clinging on somewhere on the Red Planet.
Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
