f nucleotides and amino acids.[253] The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.[254] For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analysing the few areas where they differ helps shed light on when the common ancestor of these species existed.[255]

Evolution of life
Main articles: Evolutionary history of life and Timeline of evolution


Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.[256] The three domains are coloured, with bacteria blue, archaea green and eukaryotes red.
Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.[257][258] No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years.[259] The eukaryotic cells emerged between 1.6 – 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.[260][261] The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[262] Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[263]

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.[257][264] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.[265]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.[266] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen i