What’s So Special About the Earth? Well, for one thing, it was once a great deal smaller, a solitary planet with no moon, until it served as a target for a Mars-sized object. Billiard balls might collide and rebound, but when solid objects the size of planets crash into each other, the effect is more of a molten splash. Earth ended up about 90 percent the size it is today (with more matter, including lots of water, to come in countless minor collisions later). It also ended up with the relatively rapid day-night rotation we enjoy today (although it was quite a bit faster originally), a molten iron heart that powers what amounts to a magnetic force field guarding us from the atmosphere-destroying blowtorch of the solar wind, and a sister-planet (the Moon) that simultaneously helps power active tides and serves as a gravitational stabilizer keeping the Earth’s slightly-tilted rotational axis (the bit that gives us our relatively mild, life-permitting seasons) from varying wildly.
Gribben adds that the evidence suggests the plate tectonics of Earth – the motion of crustal plates that thrusts land up above the surface of the oceans – was made possible by the thin crust that was one result of that massive collision. There’s enough water on the planet that if there were no continents, if the solid part of our world were just a quiescent round ball, water would cover it to a depth of 3 kilometers. Meaning: No land life. The pinnacle of sluggish evolution in that unchanging wet desert of ocean might be jellyfish, or bacteria. Or nothing.
Gribben goes on:
… what this really means is that the single most important factor in making the Earth a suitable planet on which a technological civilization could evolve is the Moon. And Moons like ours are really rare.
A search with the Spitzer infrared space telescope found evidence that might indicate the existence of such a moon in the (possible) solar system of only one of four hundred possible candidate stars.
What’s So Special About the Cambrian Explosion?
Imagine that you stand at the beginning of life on Earth, right where and when the first reproducing cells came into existence, and picture the blithe, confident feeling you’d have as you say, “Oh, intelligent life will always show up sooner or later.”
And then wait 3 billion years for something — anything! — interesting to happen. But nothing does. For 3 billion years. If every year was an inch, that length of time would stretch for more than 47,000 miles.
Seems … odd, don’t you think? I mean there was all this life on Planet Earth, and the same land and ocean and weather and stuff and three billion years passes with nothing to show for it but wimpy little single-celled thingies.
Near the end of the Precambrian, living things finally seemed to evidence some minimal ambition, inventing multicellularity, before finally taking off in interesting ways in the Cambrian explosion (a change of affairs that was probably driven by yet another unlikely accident).
There’s a point here that Gribben makes gently, but that I think should be made more forcefully: If there were any sort of linear inevitability to the evolution of life, some kind of general rule about growth and complexity you’d expect it to be obeyed on every planet with life. So that within a few million years of its appearance, life would smoothly progress into all sorts of cool and freaky new forms.
But it would have happened here too, don’t you think? The one sample of evolution we have seems to show that life comes with no guarantee of complexity. Not for 3 billion years, maybe not ever.
Of course, in our case, there were a few snags along the way. For instance, the Sturtian glaciation, which lasted, Gribben says “for at least 5 million years, and extended to the equator.” (See Wikipedia: Snowball Earth)
Turning away from thoughts about life for a moment, picture this: The entire land surface of the planet covered in ice. The surface of the oceans frozen, right into the tropics. All of that white, white ice reflecting much of the solar radiation right back into space. Once the process got going, for whatever reason, it would, well, snowball, turning Earth into a planetary deep-freezer that would lock in the conditions indefinitely.
The saving grace of Earth may have been volcanoes. Erupting and releasing carbon dioxide that gradually, over millions of years, thawed the deep-freeze and allowed life, which had somehow held on, another go.
But how common can all this be? A planet with a hot core capable of producing molten rock? With a thin-enough crust to allow it within spitting distance of the surface? With plate tectonics rowdy enough that would move that crust around enough for weak spots to develop and sprout CO2-emitting volcanoes? Off and on for millions of years? Hmm.
[ Continued in Part 4 ]