Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of the universe in which there are far more galaxies than people —Carl Sagan

Preamble Earth is no ordinary planet. Indeed, life on Earth is clearly beneficiary to a remarkable convergence of physical, situational and historical factors that, taken together, seem to suggest that its origin nearly four billion years ago was sui generis, one of a kind.

First Approximation

ver since 1543, when Nicolaus Copernicus showed that Earth revolves around the sun and is therefore not the centre of the universe, scientists have tended to the somewhat opposing assumption that Earth is really just an “ordinary” planet orbiting an “ordinary” star in an “ordinary” galaxy – an assumption consistent, notice, with the notion that life is likely to be rather widespread and common in the universe.

Indeed, it was just this assumption that prompted NASA, in 1971, to launch its Search for Extraterrestrial Intelligence programme – a search effort that, half a century later, has netted no persuasive evidence of extraterrestrial life, let alone intelligent life, either in our own solar system or outside of it. From this it seems to follow that Earth may not be quite so “ordinary” after all.

Consider:

• The evolution of complex (intelligent) life has six preconditions:
  1. a rocky planet with essential elements and molecules
  2. reliable sources of energy
  3. a minimum and probably a maximum planetary mass
  4. protection from hazardous radiation and impacts
  5. a narrow and optimal temperature range at or near its surface
  6. Biospheric stability over billions of years.
• Planets that satisfy preconditions 1 and 3 are presumably not too hard to come by in at least some of the 100 billion galaxies that populate the visible universe
• Because preconditions 2, 4, 5 and 6 are likely to be satisfied only rarely, their co-occurrence on Earth underlines this planet’s quite remarkable suitability as an enduring crucible for life, as follows:
  1. Viewed at the level of the Milky Way, Earth is situated in one of a small number of life-conducive sweet spots in a galaxy largely inimical to life.
  2. Our sun is a life-conducive single star, different from life-inimical double stars more prevalent in our galaxy.
  3. The gravitational reach of Jupiter has largely shielded Earth from cometary bombardment for the past 4.5 billions years.
  4. An early impact by a large planetesimal at just the right relative velocity and angle resulted in an abnormally large iron core that generates a powerful magnetosphere protective of life.
  5. The same impact also resulted in an abnormally thin crust that favours the movement of tectonic plates conducive to high topographic and ecological diversity.
  6. It further endowed Earth with an abnormally large moon, resulting in an optimal, life-friendly diurnal rotation, a stable axial tilt, and nutrient-cycling tidal flows in its oceans.
  7. For a summary of Gaian mechanisms by which life on Earth has sustained itself for nearly four billion years, see the tab essay on Gaia, below.
• Summing up, life on Earth is not to be taken for granted, but is properly understood as beneficiary to a near-miraculous convergence of life-favourable happenstance likely of rare occurrence in the universe at large.

Next up: Concerning Gaia