Canto: I'm still wondering about life.
Jacinta: There's a lot to wonder about.
Canto: Well, bearing in mind those rough and ready rules, what is the earliest form of life we know of?
Jacinta: Nobody knows. There are big arguments about the status of viruses, but the earliest candidate we know of, I mean the earliest fossil, was some sort of prokaryote, something like a modern cyanobacterium.
Canto: But it's unlikely that the earliest fossil was the earliest life form. I mean, I know very little about cyanobacteria, and obviously prokaryotes are much less complex than eukaryotes, but even one of those things couldn't have just - spontaneously generated. There must've been precursors.
Jacinta: Surely, but it's worth thinking about timelines here. The earliest prokaryotic fossil - the oldest know fossil of any kind - dates to 3.8 billion years ago. The earth was formed 4.6 billion years ago, and clearly it would've taken some time for it to be ready to sustain life.
Canto: Not so fast about that. We always think of life as we know it,we of little imagination. Look at the archaea so recently discovered. If life can thrive in hot volcanic springs, that certainly extends the range of possibilities.
Jacinta: Good point, but there's another interesting thing about this 3.8 billion date. Have you heard of the Late Heavy Bombardment?
Canto: Come on, forget about Armageddon, this is getting interesting.
Jacinta: No, really, what astronomers call the LHB took place just about 3.8 billion years ago - a heavy shower of meteorites in this region. All our evidence is from the Moon, which as you know is almost as pock-marked as Manuel Noriega's mug. Being a more or less dead rock, the Moon still bears the scars, whereas Earth's surface is shifting and spewing and subducting all over the place, so there's no direct terrestrial evidence of this shower, but it's a reasonable assumption...
Canto: Aha! Life from outer space.
Jacinta: Yes, well, that's one of many hypotheses. It's all very speculative.
Canto: Okay, I won't get carried away. Tell me about this cyanobacteria.
Jacinta: Uhh - do you mean the earliest fossils or cyanobacteria in general?
Canto: Dunno.
Jacinta: Well, cyanobacteria are mainly associated with marine environments, and they're also known as blue green algae. 'Cyano' comes from the Greek for 'blue'.
Canto: Right, as in Cyano de Bergerac, the guy with the big blue nose.
Jacinta: They've been around for a long time, engaging in what's called oxygenating photosynthesis to help transform our planet's atmosphere into something habitable for big beasties like ourselves.
Canto: Wow, photosynthesis, tell me about photosynthesis. But first, you mentioned blue green algae. Now, I would've thought algae were eukaryotic.
Jacinta: Ah yes, nomenclature and taxonomy, always fraught. The term 'blue green algae' came in long before those distinctions were made and so it has stuck. As to photosynthesis, that's one of those key and fateful processes that link the organic to the inorganic and make our fragile biosphere such an astonishing place.
Canto: You're astonished eyes delight me Jacinta, let's shore up our fragile relationship for a while before you reveal all.
Jacinta: Okay, let me reveal all before we begin. Symbiosis here we come.
Sources:
Frank Ryan, Darwin's blind spot: evolution beyond natural selection. Thomson Texere, 2003
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