Water is indispensable to life in its liquid form, and all living cells are made up of water: a human baby at birth contains 75% water, adults between 60 and 66%, and a human brain 80%!
Life was born in the ancestral ocean at around 3850 Ma, from pre-biotic chemistry, and went on to live a long, single-cell life without sexual reproduction for hundreds of millions of years. It was in the ocean that the four major biological events took place: the appearance of the nucleus and the eukaryotic cell around 2200 Ma, the capture of ambient cyanobacteria and differentiation into mitochondria and plastids (2100 and 1400 Ma), the appearance of multicellularity around 2100 Ma and, finally, the development of sexuality around 1500 Ma.
The mechanisms of speciation were described, as well as the notion of species, which is now more blurred in the light of modern methodologies. Intense speciation has always occurred during major external changes, according to the famous "punctuated equilibrium" theory of Gould and Eldredge (1993)[1]. In bacteria, the notion of species is not obvious, not to mention viruses, which evolve so much faster. Yet we must never forget the prevalence of micro-organisms on this planet: bacteria and viruses are everywhere, except in the lava of volcanoes.
[1] Gould S. J. and Eldredge N., 1993. "Punctuated equilibrium comes of age", Nature, vol. 366,no. 6452 , 223-227, DOI: 10.1038/366223a0; Boeuf G., 2013b. "Que retenir des grandes crises?", DocSciences, CRPD de l'Académie de Versailles/Muséum d'histoire naturelle (MNHN), vol. 16,no. 2 , 16-25.
