Ever wanted to know how life got started? Me too, unfortunately I don’t know, and no one else does, yet.
But we do know a few things about what’s happened to life since it got going though, and here are the highlights.
A quick note before we begin, I’ll give approximate times for each event in billions or millions of years and I’ll use the following labels, Ga for billion (giga-annum) and Ma for million (mega-annum). As millions and billions of years can be hard to imagine, I’ll also express the time in terms of a 24-hour clock, with time on Earth beginning at 00:00 and 23:59 being today.
The Earth is born; (4.55 Ga) 00:00 – rocky planetesimals condense out of a cloud of dust and gas; they grow larger, at times colliding with each other to form bigger planetesimals. As a few grow larger, their greater gravitational attraction causes them to attract more material and they grow more quickly than the smaller bodies. Eventually only a small number of large planetary embryos remain, these continue to collide and accrete until they form the four terrestrial planets; Mercury, Venus, the Earth and Mars. The Earth is a molten ball of lava. Geologists call this time the Hadean (it means Hell-like).
The Moon forms; (4.53 Ga) 00:09 – a Mars-size proto-planet called Theia collides with the young Earth. It fractures, some of its mass is incorporated into the Earth, the rest forms a smaller planetary body orbiting the Earth, the Moon. Both the Earth and the Moon continue to be bombarded by asteroids and comets.
Asteroids and comets rain on the Earth; (4.1 to 3.8 Ga) 02:25 to 04:00 – the Earth experiences a period of increased bombardment, called the Late Heavy Bombardment period. The Earth is an inhospitable place; impacts and volcanoes may prevent early life from emerging. Despite this, the Earth now has a solid crust, and the bombarding comets likely bring water to Earth. The stage is set for life to emerge.
Life emerges; (3.8 Ga) 04:00 – geological evidence suggests single-celled life emerged early in Earth’s history (although not everyone agrees on the exact timing), likely in the form of extremophile bacteria or archaea. No one knows exactly where and how this life emerged, but one candidate is thermal vents at the bottom of the oceans. Photosynthetic bacteria (cyanobacteria) and more complex cells (eukaryotes) evolve early in the history of life, but life remains as simple, single cells for a long time. The largest living structures are domed mixes of sediment and bacteria called stromatolites.
Earth experiences an Oxygen Crisis; (2.4 Ga) 11:19 – before this time Earth’s atmosphere contained little oxygen compared to today, and most organisms did not require oxygen, in fact it was likely poisonous to them. However, oxygen-levels were slowly building-up after photosynthetic bacteria evolved (as photosynthesis produces oxygen). Initially the oxygen bonded to rocks and metal, but a point was reached when these became saturated and oxygen began to be released into the atmosphere, and over the course of a few thousand years oxygen levels quickly rose. This event is often called a crisis as it may have killed most life on Earth, it may in fact have been the greatest mass extinction in terms of the % of life killed.
The Earth becomes a giant snowball; (2.4 to 2.1 Ga) 11:19 to 12:54 – the Oxygen Crisis may have also temporarily changed the face of Earth. Oxygen released into the atmosphere is thought to have bonded with methane, removing it from the atmosphere. Methane is a very powerful greenhouse gas; its removal reduced the Earth’s greenhouse effect and caused it to cool quickly. As the Earth cooled ice began to form, which reflected sunlight away from the Earth, causing it to cool further, causing more ice to form. This feedback loop continued until the Earth was covered in ice, hence the term Snowball Earth (although the exact extent of the cover is debated). The Earth likely experienced another mass-extinction as habitats were invaded and destroyed by the cold and the ice. But this catastrophe may have triggered a key evolutionary development…
First multicellular life evolves; (2.1 Ga) 12:54 – the first multicellular fossil organisms, Grypania spiralis, are found in rocks 2.1 billion years old. Before this time all life had been composed of single cells, but from this point onwards, over halfway through Earth’s history, life composed of multiple cells emerged. This may have been the result of an evolutionary radiation after a mass-extinction caused by the preceding Snowball Earth episode. Multicellular organisms have cells that specialise in different tasks, allowing more complex organisms to be created, which may have led to the development of another key innovation for life…
Multicellular life invents sex; (1.2 Ga) 17:40 – the first fossil organisms capable of sex were found 1.2 Ga ago, they were a form of multicellular red seaweed called Bangiomorpha. Before this time, organisms reproduced asexually, by splitting into two identical copies of themselves. But with sexual reproduction, offspring have a combination of both their parents DNA. Such mixing of genes facilitates the emergence of new characteristics that allows multicellular organisms to evolve more quickly than single-celled life. Once life had become multicellular the pace of evolution dramatically increased.
More Snowball Earth events; (750 to 635 Ma) 20:03 to 20:39 – within a period geologists call the Cryogenian, geological evidence suggests three or four Snowball Earth events may have occurred (the number of events and extent of ice coverage is debated). As with the previous Snowball Earth event, each occurrence may have spurred an increase in evolutionary change by wiping out many organisms and providing opportunities for new ones to evolve to fill the empty ecological niches. Potentially inspiring the…
Emergence of the first large multicellular animals; (580 Ma) 20:56 – the first fossils of large complex multicellular organisms and the first true animals are found from rocks of this period, such as Charnia and Dickinsonia. These animals are often called the Ediacaran fauna, named after one of the first locations in which these fossils were found, the Ediacara Hills in Australia. These are the first large organisms and animals we know of, with segmented bodies typically tens of centimetres across. The Ediacaran fauna died out around 540 Ma, potentially due to predation as more advanced and aggressive animals appear…
The first shells, legs and eyes; (542 Ma) 21:01 – The fossil record suddenly fills with a diverse range of animals, many of them with hard-parts such as exoskeletons and shells, and other important features such as legs, and even the first eyes, courtesy of trilobites. This period is known as the Cambrian Explosion. No actual explosion took place; it refers to the apparent sudden boom in the diversity of life in the fossil record. However, more recent evidence suggests the boom was actually preceded by a more steady flow of evolutionary change. It is hypothesised that hard parts such as shells evolved in defence to predation, and that these new predators may have been responsible for the demise of the Ediacaran fauna.
Next post, part 2; complex life really gets going, from the first fish to dinosaurs.