Re: what kinds of chemicals made up or were produced by living things on the early earth

Hello Nate,

Thanks for this very interesting question. I could be very brief and just say "nobody knows", but that would hardly be an answer! Here are a few things to think about.

First, we think that life has been on earth a very long time. The earth is about 4.6 billion years old and we have evidence of fossil cells from at least 3.8 billion years ago and maybe longer. So life got started on the earth really quite soon after the earth became cooler and water started to build up on the surface.

The earth at that time was very different. There was much more volcanic activity and meteorite impacts, it was hotter and the atmosphere was different. There is some argument over what the atmosphere was like, but we think that there was more methane, hydrogen and carbon dioxide. So the earliest living things had to be able to use these things to make chemicals. Today on earth, we find microbes called Archaea, which are single-celled organisms like bacteria. Many of these live in extreme environments (places which are very hot, salty, acid and so on) and they are often able to use simple chemicals in order to grow. Some of them can make methane from hydrogen and carbon dioxide. Others can break down methane and make chemicals derived from formaldehyde, which can then be turned into simple sugars. So we think that the earliest organisms were probably similar to these Archaea.

It's important to realise that all life as we know it is quite similar when you think in terms of chemicals. Every living thing uses DNA to store genetic information, proteins to perform chemical reactions and build parts of the cell, lipids (fats) to make cell membranes and simple sugars to provide energy and to make other chemical compounds. So it is likely that the early organisms used these same chemicals too. They probably started with very simple metabolisms which have become more complex with evolution, and there is a lot of interest today in trying to work out what the smallest amount of genes is that will make up a living cell. A lot of progress has been made now that we have the complete gene sequences of many microbes.

Of course, it may be that the earliest organisms used very different chemicals which were replaced later by the chemicals we see today, but we can never know this! A chemist called Alexander Cairns-Smith suggested that genes used to be made up of simple minerals like you would find in rock or clay and were replaced by DNA later. It's an interesting idea but hard to prove.

There are lots of interesting web sites on this topic. You might like to look at http://www.accesse xcellence.org/WN/NM/miller.html which is an interview with Stanley Miller, one of the experts on early life on earth. Another very good place is the Talk.Origins archive at http://www.talkorigins.org/ which is all about evolution of life on earth. And if you are interested in space, try searching the web for topics like astrobiology or exobiology or life on Europa and Mars. This should give you a lot of links about early life on earth, Archaea and life in extreme places.

I hope this helps with your question,

Neil