An Asteroid wiped out dinosaurs...

Despite the uncertainty on how life began, it is generally accepted that prokaryotes inhabited the Earth from approximately 3–4 billion years ago.^[2][229] No obvious changes in morphology or cellular organization occurred in these organisms over the next few billion years.^[230]

The eukaryotes were the next major change in cell structure. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called endosymbiosis.^[99][231] The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.^[232] An independent second engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.^[233] It is unknown when the first eukaryotic cells appeared though they first emerged between 1.6 – 2.7 billion years ago.

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.^[2][234] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.^[235]

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.^[236] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.^[237] About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals.^[238] Insects were particularly successful and even today make up the majority of animal species.^[239] Amphibians first appeared around 300 million years ago, followed by early amniotes, then mammals around 200 million years ago and birds around 100 million years ago (both from "reptile"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.^[138]