Fewer Harder Steps

Somewhere between 1.75 billion and 3.25 billion years from now, Earth will travel out of the solar system’s habitable zone and into the “hot zone,” new research indicates. … In the habitable zone [HZ], a planet (whether in this solar system or an alien one) is just the right distance from its star to have liquid water. Closer to the sun, in the “hot zone,” the Earth’s oceans would evaporate. (more; source)

Fifteen years ago, the best estimates I found were that life appeared on Earth from 0.0 to 0.7 billion years after such life was possible at all, and that simple life would only continue to be possible on Earth for another 1.1 billion years. (Earth is now 4.5 billion years old.) These two numbers seemed close enough to be consistent with a simple model of Earth being very lucky to originate intelligence life.

This simple model says that a planet goes from no life to intelligent life by passing some “hard steps,” like inventing life, sex, multi-cellular bodies, and intelligence. The system had a constant chance per unit time of completing each new step, but these chances could be very different. That is, the steps could have very different difficulties; it might be much easier to invent sex than to invent life.

Even so, I showed fifteen years ago that that if all these steps were hard, i.e., if on a random planet each step would usually take longer than the time window for life on the planet, then given that intelligence eventually appears before the window closes, the actual distribution of durations observed between the steps (and the duration between the last step and the end of the life window) would be roughly equal. (To be precise, drawn from the same distribution with a modest variance.)

A standard account of five major evolutionary events by William Schopf roughly fit this model: his durations were 0.0−0.7,0.5,0.6,0.7,1.1, and 1.7−2.4 billion years. And that longest period is one we know little about, so it might really cover two steps.

However, this new result quoted above, of 1.75 or 3.25 billion years for time remaining on Earth, makes this simple model harder to accept. And it is actually worse than quoted above. Those two numbers are from two different models of how the Sun’s brightness is expected to increase with time. But both numbers assume few clouds on Earth. If we instead assume that the fraction of Earth covered by clouds will later be 50% or 100%, then the time left for life is 5 or 20 billion years.

In contrast, a best estimate now is that life appeared on Earth from 0.0 to 0.6 billion years after it was first possible. So even the best case ratio for these durations is 1.75/0.6 = 3, and a more believable ratio is 3.25/0.3 = 10. These seem hard to accept as a ratio of typical durations drawn from the same distribution. So how can we change the model to better fit this data?

First, this pushes us to give up the idea that life evolved on Earth at all, or that the origin of life was a hard step. If life evolved elsewhere, that could give a lot more time for hard steps to be achieved. After all, the universe is now 13.8 billion years old.

Second, this also pushes us, if a bit more weakly, to give up the idea that the evolution of intelligence was a hard step. Intelligence seems to have appeared only 0.6 billion years after the appearance of multi-cellular animals, and we seem to see a somewhat steady progression in increasing brain size, in contrast to the constant random search and random success of the model.

Third, if there is a hard step associated with our immediate future, it is not of the sort in this simple model, something we keep trying until we succeed. Instead, either something will destroy us soon, or not.

Finally, there seems to be only room for one or two hard steps so far in the history of Earth. And the more that some periods require easy but long steps, the less room there is. For example, it might be that Earth had to wait for its atmosphere to slowly fill up with oxygen before key further developments could be enabled. Or it might be that multi-cellular animals just took a certain slow delay to develop large smart animals.

The fewer hard steps there are, the harder each steps must be on average. So this news suggests should increase our estimate of just how hard is each hard step.

The best candidate for a hard step in the history of life on Earth seems to be the origin of Eukaryotes. Since the oldest eukaryotic fossil is approximately 1.5 billion years old, they appeared reasonably close to the middle of the window for life on Earth.