I have been looking into the problem of the very early appearance of photosynthesis implied by a recent discovery of graphite rocks found to be biogenic (of photosynthetic origin) in Western Greenland. The graphite was dated 3.7 billion-year-old.
http://www.nature.com/ngeo/journal/v7/n1/full/ngeo2025.html
Scientists used to think that earth spent its first half a billion years (4.5-4 Ga) drenched in magma, the surface cooling down some 3.9 Ga yr ago. This was believed to be the reason why we couldn't find older rocks than 3.9 bl ago: none existed.
The geological period we're talking about is the Hadean, from Hades, god of the underworld, in reference to the hellish conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth.
A molten lava earth until 4 bl yr ago would make the appearance of photosynthesis in 3.7 bl yr ago--only 300 ml yr later--nothing short of miraculous.
But it turns out the surface of earth may have cooled quicker, in only about 100 ml yrs, with oceans and the opportunity for life to form much earlier than 3.9 bl ago. It's called the 'cool early earth' hypothesis. 'Cool' is relative here, earth was hugely active seismically and quite hot, but with a solid crust and some oceans, in this theory, rather than just molten lava all around.
It all started when Moon rocks brought back by the Apollo astronauts were dated through isotopic ratios. The data implied that most impact melts occurred in a rather narrow interval of time, around 3.9 billion yr ago... If that is true, almost all the craters on the moon would have the same age... Thus indicating a short period of intense bombardment. It was dubbed "the lunar cataclysm" at the times, and was controversial.
Still is. Some researchers argue that the apparent clustering of impact melt ages near this time is an artifact of sampling, but lunar meteorites found on earth have yielded similar melt age data...
The hypothesis is now called the Late Heavy Bombardment: an event thought to have occurred approximately 4.1 to 3.8 billion years ago, spanning the Hadean and Archean limit. During this interval, a large number of asteroids apparently collided with the early terrestrial planets in the inner solar system (Mercury, Venus, Earth, and Mars).
Some think that the similar age of the oldest surviving rocks and the "late heavy bombardment" is not coincidental. A period of intense meteorite bombardment in the period 4.1-3.8 Ga (giga-year) may have pulverized all rocks at the Earth's surface during the period. Sounds plausible, given the face of the moon...
If this turns out true, the LHB is the reason why we don't have older rocks. And therefore, there COULD have been a cool early earth before the LHB, and a few more hundreds of millions of years of evolution of proto-life, into life, and then photosynthetic life during the Hadean.
Calculations of earth accretion models have shown that the surface should have cooled in about 100 ml years... consistent with this hypothesis.
The apparent 'miracle' of early photosynthesis is explained? (or sort of...)
I wonder what FM thinks of that.