A few weeks ago I went out for a beer and some geology, and learned one reason why the sky is so dark. The beer came from The Burren, a rundown Irish bar in Somerville MA. The geology came from Prof. Andrew Knoll of Harvard and a bunch of his researchers and grad students. They were speaking as part of Science by the Pint, a nice local lecture series that provides researchers with free drinks if they tell the curious public about what they’re doing. The Burren has a large back room and a PA system, and so is a good venue. The dark sky comes from the Fermi Paradox, the question that Enrico asked: “Where are all the aliens?”
Knoll opened by talking about his work and his goal – to understand the diversity of life. There are millions of species of plants and animals around us, but that’s not what you would have seen for 90% of the earth’s history. It was only after the Cambrian Explosion of about 540 million years ago that really complex life evolved. Before that life consisted mainly of bacteria and the mysterious Ediacaran fauna. During it, all the major phyla were established. Life also became much more widespread, and started having a stronger effect on the chemistry of the Earth’s air, oceans, and rocks.
I once saw this transition myself. In 1996 I was on a geology field trip up in Newfoundland that was run for Harvard students by Prof. Paul Hoffman. It turns out that he and Knoll are old friends. I wasn’t a student, but he let several hangers-on like me tag along. We drove all over the province being geo-tourists. At Fortune Bay we went scrambling along the sea cliffs in search of the Cambrian Reference Stratotype. This is the rock layer that defines the start of the Cambrian epoch. About halfway down the cliff we found it – a thin yellow line among the strata. Below it was only sand. Above it, it was full of shells. The start of complex life, right there. That was Genesis, facing us in the rock.
But why then? There are scads of theories, but Knoll thinks it’s due to a profound chemical change that was happening then – the rise of oxygen levels. There was an early pulse of oxygen production about 2 billion years ago called the Great Oxygenation Event, but then there were wild swings in its levels, as seen by the deposition of oxides of chromium 63. By the time of the Cambrian it was still only up to maybe 10% of its current level. The O2 level may have been rising because more organics were dying and taking their carbon down to the sea floor. Or it could have been going up because more creatures were burrowing into the sediments, releasing sulfur, a key element for life, and so enabling more photosynthesis.
Without O2 it’s hard to imagine how big multi-cellular creatures can grow. Bacteria can live without it, but there are no large creatures at all that use anaerobic metabolisms – they all need oxygen for the ATP cycle that provides energy to cells. They need a lot of oxygen at that, since it has to diffuse directly into the tissues of primitive animals instead of being moved around by circulatory systems. Furthermore, you need ozone generated from O2 to block the lethal amounts of ultraviolet produced by the sun.
So O2 rises, and suddenly all the genetic experiments that had been happening for the previous hundreds of millions of years have a chance to thrive. The fundamental building block of complex life, the eukaryotic cell, was already around by maybe 1.6 billion years ago, but couldn’t glom together with a lot of other cells until there was enough O2 for all.
How does all this relate to aliens? Because it looks like the earth itself is not all that hospitable to creatures like us. The earth is about 4.5 billion years old, and has had life for maybe 3.5 billion years of that, but that life was pretty much pond scum. It’s only in the last 500 million years that complex life was possible. It’s only in the last 400 million that anything could survive on land. Even within that span it’s pretty likely that no other species ever got to our technological level. We’ve already burned up maybe 30% of the planet’s coal, which was mainly laid down about 350 million years ago. Any other industrial species would have taken it all long ago.
So the most bio-friendly place that we know of in all the universe isn’t actually all that nice. It was uninhabitable for 90% of its history. It was uninhabited by tool users until a million years ago. It’s no wonder that Enrico Fermi didn’t see the contrails of fusion-powered starships criss-crossing the galaxy! Life may arise easily, but complex life needs a lot of other factors to come together in just the right way. Those complex lifeforms can now have a pretty pleasant existence, hanging around with each other drinking fermented plants and talking about rocks, but they’re likely to be rare in the universe.