It took 3 or 4 billion years to produce evolution Homo sapiens. If the climate had completely failed once at that time, then evolution would have stopped and we would not be here now. So, in order to understand how we came to exist on planet Earth, we will need to know how the Earth managed to stay in shape for life billions of years.
This is not a trivial matter. Today’s global warming shows that the climate can change considerably over the course of centuries. Over geological time intervals, it is even easier to change the climate.
Calculations show that there is the potential for the Earth’s climate to deteriorate to freezing or boiling temperatures in just a few million years.
We also know that the Sun has become 30% brighter since life evolved. In theory, this should have caused the oceans to boil by now, given that they were not generally frozen on early Earth – this is known as the “paradox of the weak young man”. However, somehow, this housing puzzle has been solved.
Scientists have come up with two main theories. The first is that the Earth could have something like a thermostat – a feedback mechanism (or mechanisms) that prevents the climate from ever wandering at fatal temperatures.
The second is that out of a large number of planets, some may be lucky, and Earth is one of them. This second scenario is made more plausible by the discoveries of the last decades of many planets outside our solar system – the so-called exoplanets.
Astronomical observations of distant stars tell us that many of them have planets that orbit and that some have a size and density and an orbital distance so that life-appropriate temperatures are theoretically possible. It has been estimated that there are at least 2 billion such candidate planets in our galaxy alone.
Scientists would love to travel to these exoplanets to investigate whether any of them have matched billions of years of Earth’s climate stability. But even the nearest exoplanets, those orbiting the star Proxima Centauri, are more than four light-years away. Observational or experimental evidence is hard to find.
Instead, we explored the same question by modeling. Using a computer program designed to simulate the evolution of the climate on the planets in general (not just on Earth), we first generated 100,000 planets, each with a different random set of climate feedback. Climate feedbacks are processes that can amplify or diminish climate change – consider, for example, melting sea ice in the Arctic, which replaces ice that reflects sunlight with the open sea that absorbs sunlight, which in turn causes more more heating and more melting.
To investigate how likely each of these various planets remained habitable on enormous (geological) scales, we simulated each 100 times. Each time the planet started from a different initial temperature and was exposed to a different random set of climatic events.
These events are climate-changing factors, such as the eruption of the supervolcano (such as Mount Pinatubo, but much larger) and the impact of the asteroid (such as the one that killed the dinosaurs). On each of the 100 races, the planet’s temperature was tracked until it became too hot or too cold or otherwise survived for 3 billion years, at which time it was considered a possible crucible for intelligent life.
The results of the simulation give a clear answer to this habitability problem, at least in terms of the importance of feedback and luck. It was very rare (in fact, only once in 100,000) for a planet to have stabilizing feedback so strong that it remained habitable 100 times, regardless of random climatic events.
In fact, most planets that have remained habitable at least once have done so less than ten times out of 100. Nearly every opportunity in the simulation when a planet has remained habitable for 3 billion years has been partly lucky.
1,000 different planets were randomly generated and run twice. Green circles show habitability for 3 billion years. (Toby Tyrrell)
At the same time, luck itself proved to be insufficient. Planets specially designed to have no feedback at all have never been habitable; random walks, hit by climatic events, never lasted the course.
This overall result, that the results depend partly on feedback and partly on luck, is robust. All sorts of modeling changes did not affect her. By involvement, the Earth must therefore possess some climate-stabilizing feedback, but at the same time, luck must have been involved in remaining habitable.
If, for example, an asteroid or a rocket were a little bigger than it was or took place at a slightly different (more critical) time, we probably wouldn’t be here on Earth today.
It offers a different perspective on why we are able to look back on the remarkable, extensive, history of life on Earth, which is evolving and diversifying and becoming increasingly complex to the point where it gave birth to us. 
Toby Tyrrell, Professor of Earth System Science, University of Southampton.
This article is republished from Conversation under a Creative Commons license. Read the original article.