Re: Could there be life on Earth after humanity is gone?

Dear Julianne,

Thanks for your intriguing question. I should say at the outset that this kind of question is very difficult to answer. We might call this field "futurology". Science is very good at making short-term predictions where we know all of the variables, but noone can predict the future of planets or species. What we can engage in is some fun speculation and a few pointers to interesting resources.

The first point that I'd like to make is this: assuming that the earth is not completely vaporised or transformed into an incredibly hostile environment (hundreds of atmospheres pressure, hundreds of degrees temperature, corrosive acidic atmosphere - like Venus), there will most certainly be life here long after humans are gone. We can be confident of this because there was life on earth long before humans arrived! In fact, modern humans have existed for only a few hundred thousand years, whereas the first evidence of cells is from over 3 billion years ago. I think it's important to remember that humans are not the pinnacle of evolution - there have been and will be millions of other species on earth.

The second point to think about is that life on earth has survived far worse catastrophes than a few degrees of warming in its long history. There have been collisions with comets and asteroids that have blotted out sunlight for years on end, acidified the oceans and wiped out up to 90% of species living at the time. There have been natural climate extremes from global warming to global ice ages. Around 2 billion years ago, a new toxic gas appeared in the atmosphere that was deadly to many early species - the gas was oxygen, generated by the first photosynthetic cells! Through all of this, a few species have always survived. Environmental and genetic variation then ensure that new species will evolve and repopulate the earth through the process of evolution by natural selection.

So to answer your question: "could another lifeform grow and survive on our planet long after we're gone?" - yes it could and they are all around us already. As to whether a new species could emerge and replace the human race - that is, colonise the planet to the extent that we have - who knows? In a way, this would be a giant experiment in evolution where we wind back the clock and start again. It's unlikely that we would end up with the same species that we see today because we would never repeat the pattern of environmental selective pressures that have occurred in the time since the earth formed and life began. Perhaps there'd be a race of super-intelligent insects? Or nothing more than green slime coating the planet? Nobody can say.

A couple more thoughts. Proponents of an idea called panspermia suggest that life originates out in space. So perhaps a dead earth could be reseeded with new, exotic alien life. It's nice to think (though there's no evidence so far) that there are other intelligent races in our galaxy - might they recolonise the earth, assuming that they ever figure out how to get here? And finally - they'll have to be quick, because the earth has a finite lifespan. If our sun behaves in the way that other similar stars do, it will begin to run out of fuel in around 5 billion years time, expanding into a red giant star and consuming the inner planets - perhaps including the earth - in the process. So if our race, or any other race that colonizes the earth is to last for billions of years, it won't be on the earth - they'll have to move out into the solar system and beyond.