Recently, a reader asked about the piece, ‘Earth’s first deep breath’. The assertion that the atmosphere contained much less oxygen 2.5 billion years ago raises the question of how did we get at this number? 2.5 billion years is a long, long time ago. Can these measurements be repeated? Is there a standard?

If a species lived 500 million years ago, and the evolutionary path branched at some particular point, are these numbers accurate? When dinosaurs were wiped out about 70 million years ago, how come species like crocodiles, snakes, etc. manage to survive?

Some answers:

The first question is on how the figure 2.5 billion years was arrived at. As the article said, the dates were fixed with the help of biochemical clues and geological evidence. There is an extensive body of knowledge in these areas. Atmospheric levels of trace elements like radioactive carbon are promptly replicated in living things, which incorporate these elements in their tissues in the same proportion as in the atmosphere.

If the living thing should die and get preserved, it stops exchanging its composition with the atmosphere to adjust to the same levels, and the living thing's remains become a sort of permanent record of the proportion of elements in the atmosphere at the time the specimen was alive.

Another permanent record of ancient times is in the ice that is preserved deep under the polar snowcaps. Ice formed from the rainfall of each era captures samples of water vapour and air of the time.

Another measure is the molecular clock. There is evidence of the speed of evolutionary changes in certain convenient features of living things. Where such changes are also marked by fossil or other evidence, it is possible to calibrate genetic changes against time.

Your question about how some reptiles survived when the dinosaurs were wiped out. The argument is that dinosaurs had evolved to become highly dependant upon the existing environment and hence could not survive the changes that were wrought by events about 70 million years ago. So the dinosaurs bowed out, but not all creatures.

(The writer can be contacted at simplescience@gmail.com)