A paper recently appeared in Nature Geoscience that may have found the oldest case of mass murder by poisoning. Think of ancient cases of poisoning and your mind might go back as far as the ancient Greeks and their inclination towards hemlock (the Greek philosopher, Socrates died of hemlock poisoning in 399 BCE). But the ancient Greeks were just yesterday compared to the latest ideas around mass extinctions some 3- billion years ago.
The early cyanobacteria developed the neat trick of combining water and carbon dioxide to make sugars and the other molecules of life, such as amino acids and proteins. To do this, the cyanobacteria needed energy and they got that from sunlight. This neat trick is still around to this day and we call it photosynthesis. As the cyanobacteria started to photosynthesise, so they became the antisocial life-form from hell, for they produced a deadly poison – oxygen.
To us oxygen is essential for life but the early earth was devoid of atmospheric oxygen. Three billion years ago the atmosphere may have contained one ten thousandth of a percent oxygen, compared with today’s value of close to 21%. Oxygen however, is very reactive and strips electrons from organic molecules. Life 3-billion years ago was anaerobic and the buildup of oxygen in the atmosphere represented a real threat. Existence of animal life today is only possible because of some intricate biochemistry that protects from oxygen toxicity.
The 3-billion year old mass poisoning referred to here however, was not caused by oxygen – in fact just the opposite. An ability to utilise oxygen to generate energy from food is a much more efficient processes than anaerobic metabolism. Evolutionary pressure favoured those early organisms that evolved ways of harnessing oxygen and by two an a half billion years ago, oxygen-utilising life was the norm. Timescales raises a question however, and that is why did is take 500 million years for oxygen-based life to become dominant. The answer may well be iron toxicity.
The theory is that volcanism boiled up large amounts of iron from the inner core of the earth, which ended up in the archaean oceans. Iron exists in different oxidation states of which two dominate, know as Fe2+ and Fe3+. Since electrons are negatively charged, as iron atoms lose electrons, so they gain a positive charge. If the iron atoms loses 2 electrons, it gets two positive charges (Fe2+) and if it loses 3 electrons then it gets three positive charges (Fe3+). One good way of losing an electron is by reacting with oxygen. As iron poured into the the archaean oceans so it reacted with the emerging oxygen preventing the buildup of the gas in the atmosphere. Presence of Fe2+ was also toxic to much of the emerging life and so evolution of complex life was inhibited by half a billion years. When the amounts of oxygen eventually overwhelmed the available iron, so atmospheric oxygen rose, in the great oxidation event that took place around 2.5 billions years.
Interestingly without volcanic iron, life on earth (and by inference ourselves) would be half a billion years older. By now we may have evolved beyond being human, we may have colonised the stars or we may have had time to become extinct – who knows?
Graham Lappin 