MadSci Network: Evolution

Re: Is chemosynthesis older than photosynthesis?

Date: Thu Mar 16 13:23:34 2000
Posted By: Neil Saunders, Post-doc/Fellow, Molecular Cell Physiology, Vrije Universiteit
Area of science: Evolution
ID: 953094577.Ev

Hi Raul, Thanks for this question-it is a good one. Here is the short answer: yes, chemosynthesis is a more viable option for early life and yes, we believe that it predates photosynthesis. Now for the longer answer!

There is a lot of debate about the composition of the early earth. It was hot and therefore volcanically active. There would have been large out- gassings of water vapour into the atmosphere and frequent electrical storms, all as depicted in the pictures that you mention. In the 1950's, biologists Harold Urey and Stanley Miller tried to simulate the early atmosphere by passing electrical discharges through mixtures of water vapour, methane and ammonia, and they showed that many basic biochemicals could be formed in this way, including sugars, amino acids and the bases of nucleic acids. These experiments have shaped the way that scientists think about how life started ever since.

The experiments assumed that the early atmosphere was reducing-that is, rich in hydrogenous compounds such as methane. Since then there has been much debate about this, with many scientists now believing that the atmosphere was less reducing than previously assumed. However, one thing we are quite sure of is that the early atmosphere was not oxygen-rich. The oldest known rocks contain very low amounts of "banded iron ores", formed when iron reacts with free oxygen. These types of ore increase over time, starting about 2.5 billion years ago. So it seems that free oxygen appeared and increased over time, and the only source of oxygen that we know for this is photosynthesizing micro-organisms.

But we have fossil evidence for simple, single-celled organisms long before this time. They must have had very simple metabolisms and the simplest that we know of are chemosynthetic. They were probably similar to the kingdom of micro-organisms called Archaea, that exist today. They often live in extreme environments and can derive energy by oxidising simple compounds or even elements, like iron or hydrogen. Photosynthesis had to wait for 2 things: (1) the production of chlorophyll, or a similar compound, that could capture energy from sunlight, split water and generate hydrogen and (2) the ability to combine that hydrogen with carbon dioxide to make sugars. But once photosynthesis was established, the organisms that could perform it spread to take over the earth. Of course, the new oxygen was toxic to the previous inhabitants and they were banished to the extreme corners of the earth for ever! And in time, organisms evolved that could tolerate and use the oxygen.

This is a vast and fascinating topic, with many unanswered questions and contentious areas. If you want to read more, a good discussion of the early earth can be found in a great book, "Life: An Unauthorised Biography" by Richard Fortey. There are also excellent resources at:

This is the Talk.Origins archive, which discusses many evolutionary topics, including the early earth.

Neil Saunders

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