Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 129

the means of energy and the vital materials to survive and replicate,
and it is the medium in which DNA, proteins, and other components
of the cell operate. It is directly involved in primary energy storage
from the Sun, as the process of photosynthesis builds sugars from
water and carbon dioxide, and yields these again when the sugars are
respired for energy. At some point, certainly as early as 3.8 billion
years ago and possibly somewhat earlier, life took off. This is evident
in the rock record from black organic-rich shales with iron pyrite
(fool's gold) that signal the activity of early sulphur-utilizing microbes.
These earliest organisms might have been the Archaea, a group that
resemble bacteria in small size and 'simplicity', but which have—
possibly—yet more ancient origins.
The Engine of Life Turns Over
We cannot see clearly from the fossil record how life first evolved, but
we do know that all cells only grow and replicate if there is a supply
of energy. On Earth, life is ultimately sustained by the heat and light
of the Sun, and to a much lesser degree—at present—by the energy
released by radioactive decay within the Earth's interior, an internal
heat that is vented by volcanic activity at the surface. In the very deep
past, though, the Earth was hotter, and it is plausible that life origi-
nated in waters connected to chemical energy from rocks. This is
even more plausible if one thinks about the intense meteoritic bom-
bardment of the early Earth, which continued into the Late Heavy
Bombardment, and which could have resulted in either the complete
annihilation of early surface oceans, or at the very least (for projec-
tiles 'only' 300 kilometres in diameter) left behind only a boiling
watery residue. 77
Amid such impacts, early life might have clung on below the sur-
face, in pockets of water between the rocks more than a kilometre
below the surface. Analysis of the time it took for the Earth to cool