Beginning of Life
The first life may have developed in undersea alkaline vents, and was probably based on RNA rather than DNA.
At some point far back in time, a common ancestor gave rise to two main groups of life: bacteria and archaea.
How this happened, when, and in what order the different groups split, is still uncertain.3.5 billion years ago
The oldest fossils of single-celled organisms date from this time. 3.46 billion years ago
Some single-celled organisms may be feeding on methane by this time.3.4 billion years ago
Rock formations in Western Australia, that some researchers claim are fossilised microbes, date from this period.3 billion years ago
Viruses are present by this time, but they may be as old as life itself.2.4 billion years ago
The “great oxidation event”. Supposedly, the poisonous waste produced by photosynthetic cyanobacteria – oxygen – starts to build up in the atmosphere. Dissolved oxygen makes the iron in the oceans “rust” and sink to the seafloor, forming striking banded iron formations.
Recently, though, some researchers have challenged this idea. They think cyanobacteria only evolved later, and that other bacteria oxidised the iron in the absence of oxygen.
Yet others think that cyanobacteria began pumping out oxygen as early as 2.1 billion years ago, but that oxygen began to accumulate only due to some other factor, possibly a decline in methane-producing bacteria. Methane reacts with oxygen, removing it from the atmosphere, so fewer methane-belching bacteria would allow oxygen to build up.
2.3 billion years ago
Earth freezes over in what may have been the first “snowball Earth”, possibly as a result of a lack of volcanic activity. When the ice eventually melts, it indirectly leads to more oxygen being released into the atmosphere.
2.15 billion years ago
First undisputed fossil evidence of cyanobacteria, and of photosynthesis: the ability to take in sunlight and carbon dioxide, and obtain energy, releasing oxygen as a by-product.
There is some evidence for an earlier date for the beginning of photosynthesis, but it has been called into question.
2 billion years ago?
Eukaryotic cells – cells with internal “organs” (known as organelles) – come into being. One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA.
Eukaryotic cells evolved when one simple cell engulfed another, and the two lived together, more or less amicably – an example of “endosymbiosis”. The engulfed bacteria eventually become mitochondria, which provide eukaryotic cells with energy. The last common ancestor of all eukaryotic cells had mitochondria – and had also developed sexual reproduction.
Later, eukaryotic cells engulfed photosynthetic bacteria and formed a symbiotic relationship with them. The engulfed bacteria evolved into chloroplasts: the organelles that give green plants their colour and allow them to extract energy from sunlight.
Different lineages of eukaryotic cells acquired chloroplasts in this way on at least three separate occasions, and one of the resulting cell lines went on to evolve into all green algae and green plants.
1.5 billion years ago?
The eukaryotes divide into three groups: the ancestors of modern plants, fungi and animals split into separate lineages, and evolve separately. We do not know in what order the three groups broke with each other. At this time they were probably all still single-celled organisms.