Fermi paradox meets the timescale

This is a repost from another blog, but I found it interesting enough that I would like to share it here. Basically, it is asking why we have not seen aliens yet? And the logic is that if they visited the planet 50 times, the chance of running into us is tiny: humans have only been around for a very very short amount of time.

That said, I do have a bit of a hard time with this, because I think we are being visited constantly, and I dont really understand why everyone refuses to admit it.

Fermi paradox meets the timescale

John over at cosmic variance has been discussing extra-terrestrial life, so I figured I'd put a geologic spin on it. Specifically, look at the Fermi Paradox through the lens of deep time. The Fermi paradox states, "If advanced aliens are common in the galaxy, where are they?" More specifically, why aren't they here. As a geochronologist, I don't wonder where and why, I wonder when. So let's make a few assumptions:

Suppose that Earth has been visited by aliens 50 times since our solar system's accretion disk started to cool 4,567 million years ago. What would the aliens have seen? In order to simulate this, I generated 50 random alien arrival times in between then and now, sorted them, and put them in geologic context. They are listed below, in stratigraphic order.

Time (Ma)	Time (name)	My comment	Alien's comments
125	Cretaceous	Dinosaurs!
270	Permian	Gondwanan glaciers and funky reptiles
352	Carboniferous	Swamps and really big insects
668	Cryogenian	Pre-Marinoan- no sponges yet
675	Cryogenian
701	Cryogenian
748	Neoproterozoic
750	Neoproterozoic
808	Neoproterozoic
925	Neoproterozoic
1021	Mesoproterozoic	Grenville	Big Mountains
1049	Mesoproterozoic	Grenville	Big Mountains
1300	Mesoproterozoic
1355	Mesoproterozoic
1533	Mesoproterozoic	Mt Isa is forming	If these aliens came for resources, they didn't want base metals
1684	Paleoproterozoic
1857	Paleoproterozoic
1888	Paleoproterozoic
2159	Paleoproterozoic	Trans-Amazonian orogeny
2247	Paleoproterozoic	Various poorly constrained glaciations in this general timeframe
2272	Paleoproterozoic	Various poorly constrained glaciations in this general timeframe
2355	Paleoproterozoic
2358	Paleoproterozoic
2400	Paleoproterozoic
2459	Paleoproterozoic	Oxygen just starting to leak into the atmosphere, Manganese and BIFs
2610	Neoarchean
2612	Neoarchean
2631	Neoarchean
2661	Neoarchean
2682	Neoarchean
2745	Neoarchean
2948	Mesoarchean
2956	Mesoarchean	These two mesoarchean visitors missed each other by only 95,000 years.
2956	Mesoarchean	These two mesoarchean visitors missed each other by only 95,000 years.
2972	Mesoarchean
2990	Mesoarchean
3152	Mesoarchean
3281	Paleoarchean
3609	Eoarchean
3614	Eoarchean
3641	Eoarchean
3647	Eoarchean
3669	Eoarchean
3828	Eoarchean
3837	Eoarchean
3875	Eoarchean	LHB	Dynamical instability of this system precludes the development of complex life
3947	Eoarchean	LHB	Dynamical instability of this system precludes the development of complex life
4011	Hadean
4266	Hadean
4425	Hadean	Moon forming impact	"That's not a moon, that's a battlestation"

As you can see, for aliens looking for 'Earthlike' planets, the actual Earth was easy to overlook for msot of its history. In this simulation, there was only macroscopic life for 3 of 50 visits. From another POV, three visits were eaither during the Late Heavy Bombardment, or during the moon forming impact- both of which would appear (to the casual alien visitor) to make long-term viability of life on Earth pretty unlikely.

So as we start to find 'earth-like' planets in our sky surveys, it is important to remember that Earth has only been Earthlike for a relatively short period of time.