Planet Earth has been around for the past 4.5 billion years or so: about the last third of the Universe's history.

Through a combination of geology and paleontology, we can trace the existence of life back more than four billion years, teaching us that life arose on Earth very early on.

Life survived and thrived for billions of years before human beings arose: apparently the first intelligent and technologically advanced species on our planet?

How likely is intelligent life.

While we haven't found either beyond Earth just yet, a new study claims to have determined whether life "rarely" or "commonly" occurs on Earth-like worlds, concluding that life may be common, but intelligence is rare.

Here on Earth, life arose very early on in our planet's history, while intelligent life only arose...

This can help us estimate probabilities for whether life is common or rare, along with intelligent life's likelihood, on Earth-like worlds.

It's very difficult to know what the true probability of life arising on Earth is, or what the odds that intelligent, technologically advanced life would arise.

In order to know such a thing, what we'd ideally want to do is to create that same environment that was present on Earth back at the time of its formation many times over, watch each of those environments evolve over ~4.5 billion years, and see what comes out.

For the question of how likely life or intelligent life on an Earth-like world actually is, it's actually an impossible approach.

For starters, we only have one planet (Earth) that we know of where life exists at all, and it's not like we have Suns and Earth-like planets just waiting around for us to observe them over 4.5 billion year timescales.

The number of successes â€” whether you define success by life, intelligent life, or some other criteria â€” divided by the total number of attempts will give you your probability of success.

[+] organisms can have their lineage traced back to a universal common ancestor that lived an estimated 3.5 billion years ago, and much of what's occurred in the past 550 million years is preserved in the fossil records found in Earth's sedimentary rocks.

Just as you know the probability of getting two six-sided dice to sum up to 7 is one-sixth, you could know the probability of life (or intelligent life) arising on Earth.

[+] indicate an age of 4.26 billion years, or nearly the age of Earth itself.

We know that life arose on Earth relatively early on.

On the other hand, complex life didn't arise until shortly before the Cambrian explosion (just 600 million years ago), and intelligent, technologically advanced life only came to be with the arrival of human beings.

You can assume, as a starting point, that each of the relevant questions has a 50:50 probability, where there's a 50% chance that life arises commonly and a 50% chance that it arises only rarely.

Similarly, assuming life arises, there's then a 50% chance that it becomes intelligent frequently and a 50% chance that it becomes intelligent only rarely.

(In this case, getting life and/or intelligent life out of them in a way that aligns with what happened on Earth.) In a new paper just published on May 18, 2020, David Kipping did exactly this, providing the first robust likelihood analysis of these four scenarios.

If you go with the microfossil evidence, it gives you better than 3-to-1 betting odds that life arises commonly rather than rarely; if you use the (disputed, but still compelling) evidence from zircon deposits, the betting odds increase to better than 9-to-1 that life is common rather than rare.

If you start with the same betting odds (50:50) for intelligent life â€” assuming that life has arisen, is intelligent life common or rare â€” the Bayesian analysis that Kipping performed slightly favors the rare-intelligence scenario.

If we started with a clone of early Earth, life would likely emerge, but we cannot reach a good conclusion concerning the emergence of intelligent life.

We may truly be alone in the Universe, but the honest answer is we don't know enough about the relevant probability to say so.