Re: Similarities in the past and present environments of Archaea?

hi Rebecca,

I guess that you are referring to this previous question and my answer:
Similarites between past & present environments of Archaea?

I think that you and the previous questioner had some difficulty with the question because - it's not a very well thought out question! The problem is that the question has been phrased in a particular way for a reason, but whoever wrote the question hasn't really thought it through. Let me try to explain what I mean.

First, the question implies that archaea inhabit certain environments. This is not true. Archaea are ubiquitous - found everywhere. There are large numbers in the oceans, in soil, in aquatic environments, everywhere from the polar regions to the deserts to the guts, teeth and skin of animals - even humans.

Whoever wrote the question is thinking about those Archaea that are extremophiles. This means that they inhabit environments that we would find extreme - very hot or cold, very saline, very acidic or alkaline. Many of the microbes that live in these environments are archaea - but many are also bacteria. In other words, not all archaea are extremophiles and vice versa.

Next, we have the problem of "past environments". What does this mean? How long in the past are we talking about? My guess is that the question writer is talking about the very early history of the earth, perhaps 3.5 to 4 billion years ago when we believe that the first cells appeared. Their reasoning then goes: the earth back then was more "extreme" - it was hotter, with more volcanic activity, the composition of the atmosphere was different (no oxygen, more methane, sulphurous compounds and so on) and so single-celled organisms would have been what we call extremophiles and probably resembled the archaea of today. The implication is that today, archaea tend to be found only in extreme niches, whereas in the past they would have been more widespread.

Well as we just saw, archaea are widespread - because they are not all extremophiles! Furthermore there is very little fossil or molecular evidence to tell us whether early cellular life resembled archaea and there is a great deal of debate about the exact conditions on the early earth. Really, all the evidence that we have comes from phylogeny - the use of gene sequences to infer relationships between organisms. When we look at phylogenetic trees we see that (1) archaea are separate from bacteria and eukarya and (2) thermophiles (organisms that live in hot places) are found close to the "roots" of the tree. That places them close to the common ancestor of todays organisms, so it's generally believed that early microbes were thermophiles. And therefore possibly - but not necessarily - like archaea.

I hope that this has made things clearer for you. In science, it's important to phrase your questions carefully and precisely if you want to get an answer. Next time you are searching Google, don't limit yourself to search terms like "archaea" - try "early earth", "astrobiology" and anything else related to the topic.

Last, here are a few Archaea links for you:

ArchaeaWeb
Introduction to the Archaea
Domain Archaea
History of extremophiles
Another introduction to Archaea
Archaea at Wikipedia