Life takes its time and at certain moments accelerates. This phrase could very well synthesize the evolution of life on Earth. The pace of change is slow but sometimes it gets hectic. If Nature had a conscience, we could say that sometimes she is in a hurry.
The first of these moments occurred at the beginning. Life appeared on our planet very early , between 500 and 700 billion years after the formation of the Earth. And it did not do it before because it would have been impossible: during those first hundreds of billions of years the Earth was subjected to an intense bombardment of comets and meteorites that sterilized the planet’s surface. In fact, our beloved Moon arose from one of these impacts against a body the size of Mars.
Ironically, this rain of fire and rocks helped life to appear: on the one hand, they provided a large part of the basic bricks for life and, on the other hand, they deposited between 30 and 50% of the water that we have today.
Four billion years ago, most of the Earth’s surface was a boiling ocean . High volcanic cones spewed large amounts of gases into a dense, unbreathable atmosphere. The sky was almost completely free of clouds and the Sun flooded the Earth with its light and its deadly ultraviolet rays. Inland, the scene was dominated by mounds of black lava whose surface was covered in debris. Everywhere there were pools, shallow and very saline. Perhaps in those pools the first forms of life arose . Or perhaps it was in certain places on the bottom of the oceans, where the lava from the interior escaped through a crack in the thin oceanic crust.
unicellular life
If we had to point to the place where the oldest fossil evidence is found, we would have to look towards the North Pole. But not the one we all know, but the one in the inhospitable Pilbara region, in Western Australia , and named after gold prospectors. There, where temperatures exceed 50ºC, we have found evidence of life on Earth 3.5 billion years ago. What’s more, the rock formation known as the Warrawoona Group contains four different types of microfossils. In these rocks we find the stromatolites, subtle indicators of what the Earth was like in the past and proof of the important role that microbes played billions of years ago.
Stromatolites are not fossils in the strict sense of the word. They were produced in the primitive oceans of the Earth due to the metabolic activity of certain microorganisms called cyanobacteria. These bacterial communities built the stromatolites in the ocean by trapping the dust that makes up the finer sediment with a layer of sticky mucus secreted by the cell itself, building the rock from the inside out. Since the cyanobacteria were photosynthetic and capable of moving towards the light, they were always found on the outer surface of the stromatolite.
They grow very space: little less than half a millimeter per year . It takes hundreds of years for these dome-shaped structures to reach 12 inches tall and 8 inches wide, which doesn’t sound like much. But it begins to be if we take into account the size of the bacteria that formed them, with a size of a few millionths of a meter. On a human scale it would be equivalent to building a structure 105 kilometers high by 75 kilometers wide.
The end of a long reign
Stromatolites dominated the planet for 3 billion years. When the great explosion of marine life was experienced on Earth, 600 million years ago, its star began to decline . They had lived everywhere from the glacial lakes of Antarctica to the volcanic springs of Yellowstone, but they couldn’t handle the new environment that was emerging. With them, that old aphorism that nothing lives forever was also fulfilled.
But the most fascinating thing about their life is that even today, despite the profound changes that have occurred on Earth, we can still find them alive on the western coast of Australia, in Hamelin pool, in Shark Bay . How are they still living? These bacterial colonies, the oxygen factories that we breathe today, are very fragile. Some scientists think they survive because there are no predators, such as gastropods, to eat them. However, it is more likely because they live in an environment with few nutrients and it is not an attractive place for potential competitors.
References:
Allwood, Abigail C.; Walter, M.R.; Kamber, B.S.; Marshall, C.P.; Burch, I.W. (2006). “Stromatolite reef from the Early Archaean era of Australia”. Nature. 441 (7094): 714–8. doi:10.1038/nature04764
Stinchcomb, B. and Leis, R.J. (2015) Stromatolites: Ancient, Beautiful, and Earth-Altering, Schiffer
