University of Bristol


clock The Earth is 4.5 billion years old (give or take 200 million years!) formed from a rotating disk of gas and dust in the young Solar system. The earliest evidence we have for life on Earth comes from rocks 3.75 years old in Greenland. clock
These sedimentary rocks record the cooling and stablisation of the Earth's crust, and contain a ratio of 12C/13C isotopes which indicate an organic origin for the rock carbon. In other words, life had originated nearly 3.8 billion years ago, about as early as was possible considering the time required for the young Earth to cool.

The earliest morphological evidence for life is 3.5 billion years old, fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes. For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates. It was another 1.4 billion years before complex, multicellular life made an appearance in the form of the Ediacaran faunas (see fossils), followed by all the variety of the true Cambrian animals about 550 million years ago.

Therefore 80% of the history of life on Earth is exclusively single or undifferentiated multi-cellular. 3 billion years went by before complex multicellular life appeared, but when it did it only took between 5 and 10 million years for all the basic body plans of the organisms we see around us today to be established. This is why the origin of multicellular life, in particular the metazoans or large animals with complex body plans, is termed the Cambrian explosion.