It's alive – isn't it?
Speculations on the origin of life have, until recently, been purely theoretical. The latest experimental findings, however, are raising more questions than they're answering. Jon Evans reports
Astronomers believe that the Earth was formed, along with the rest of the solar system, around 4600m years ago. For the next 500m years the Earth suffered under an intense bombardment of comets and meteorites, the leftover debris from the formation of the solar system, which kept the Earth's surface in a constant state of molten upheaval.
However, the fossil record shows that a mere 600m years after the bombardment stopped (ie around 3500m years ago), organisms very similar to present-day cyanobacteria (blue-green algae) abounded in the Earth's seas. Indeed, some forms of life may have existed even earlier: rocky sediment from Greenland dating back 3800m years is enriched with the isotope 12C, a sign of the biological processing of carbon (inorganic carbon chemistry does not distinguish 12C from 13C, whereas biological processes preferentially use 12C).
So life may have been present only 200m to 300m years after the Earth could first possibly have supported it. And, perhaps even more astonishingly, there is fossil evidence that life as we would recognise it today, probably using DNA and proteins as its basis for replication and metabolism, was thriving only 300m years later (about 3500m years ago).
This all points to the possibility that the formation and development of simple life was not an especially difficult process; at least not when compared to the evolution of land plants, which only appeared some 3000m years after the ancestral cyanobacteria. It seems as though simple chemical reactions among the compounds that scientists envisage were available on the early Earth were sufficient to drive the process that led inexorably to life (see Box 1).
There is, however, one problem with this assumption: scientists still have no firm idea of what the mechanics of this kind of 'prebiotic' chemistry were or how the whole ascent to life actually happened. Recently, new experimental methods have shed some light on the possible processes, but have also served to add to the general uncertainty. One thing that scientists working in this field do know, however, is that whatever form early life took it didn't possess DNA or proteins.
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