<P> Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them . The evolution of photosynthesis, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga . The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism . Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions . Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants). However the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species . Bilateria, animals with a front and a back, appeared by 555 Ma (million years ago). </P> <P> The earliest land plants date back to around 450 Ma, although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 Ga . Microorganisms are thought to have paved the way for the inception of land plants in the Phanerozoic . Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event . (The long causal chain implied seems to involve the success of early tree archaeopteris (1) drew down CO levels, leading to global cooling and lowered sea levels, (2) roots of archeopteris fostered soil development which increased rock weathering, and the subsequent nutrient run - off may have triggered algal blooms resulting in anoxic events which caused marine - life die - offs . Marine species were the primary victims of the Late Devonian extinction .) </P> <P> Ediacara biota appear during the Ediacaran period, while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion . During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian--Triassic extinction event 252 Ma . During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods . After the Cretaceous--Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity . Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify . </P> <P> The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time . The Moon has the same composition as Earth's crust but does not contain an iron - rich core like the Earth's . Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing into orbit crust material that formed the Moon . Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted almost all the iron particles in the area . </P>

The first group of animals to live on land was the