Re: How long until humans become extinct?what year?

Hi Erika,

Thanks for submitting your question to the MadSci Network. Making accurate predictions about the future is really difficult, so I am afraid that there is no way that we can know the exact date when humans will go extinct. There are even some people who think that humans may not ever go extinct (if for example, we manage to leave the Earth and learn how to thrive in space), but I think that that will require a redefinition of what human means.

So, with that in mind, there are some things that we can think about to try to estimate what the "lifespan" for the human species might be. First off, we can look at the fossil record, to see what the average "lifespan" of a species has been over the history of life on Earth. While this is also the subject of some debate, it seems like many species go extinct after about 1 to 10 million years (many people use a value of 4 million years). Our species, Homo sapiens is only about 150,000 years or so old, so if we were an average species living in a typical time in the history of the Earth, we might expect to be around for at least another 850,000 years.

However (at the risk of sounding arrogant), we are certainly not an average species, and we are definitely not living in a typical time in history. Species have been going extinct much faster than average over the last few thousand years, and it seems likely that this is due to human activity. So perhaps there is a chance of us changing things here on Earth so much that we do go extinct. That seems unlikely to me, but as I said, it is difficult to make accurate predictions about the future.

Here is another way to think about the potential lifespan of our species. Given that our species is 150,000 years old, we can think about how much longer our species will persist if you and I are lucky enough to be living in the early part of humanity's time here on Earth, or if you and I are unlucky enough to be living in the last part of humanity's time on Earth.

Lets say that 95% of the "lifespan" of a species falls into a period that is neither very early in that species' time on Earth nor very late in its time on Earth (you can choose a larger or smaller number than 95% yourself to do this exercise, but scientists like to use 95%). In that case, lets say that the lucky part of humanity's time on Earth is the first 2.5% of its "lifespan", and that the unlucky part is the last 2.5%.

So, if after 150,000 years, we are living in the end of the lucky phase at the start of humanity's lifespan, we can estimate that that lifespan of our species will end (150,000 / 0.025)*0.975 years or 5.85 million years from now, giving humanity a "lifespan" of 6 million years. Surprisingly, that figure falls right into the average of 1 to 10 million years.

Alternatively, if after 150,000 years we are living in the start of the unlucky phase at the end of humanity's lifespan, we can estimate that the lifespan of our species will end (150,000 / 0.975)*0.025 years or 3,850 years from now, giving our species a lifespan of about 154,000 years. Given the number of people who are alive today (more than six billion), it seems unlikely to me that our species will go extinct in 4,000 years, but even if you assume that 150,000 years represents 99% of our species' time here on Earth, we still have a good 1,500 years left, so you probably won't need to worry about our species going extinct any time soon.

Of course, the chances that we are living near the beginning or the end of our species' time on Earth are low (we chose a chance of only 5%), so it is most likely that we are living in the "boring" 95% of our time, which means that these values (5.8 million years and 4,000 years) are really just upper and lower limits on the amount of time that we have left. Thats a pretty large interval, which speaks to my original point, that it is very difficult to make accurate predictions about the future. :)

For more information about the average "lifespan" of a species take a look at:
http://www.pbs.org/wgbh/evolution/library/03/2/l_032_01.html

Raup, DM (1978) Cohort analysis of generic survivorship. Paleobiology, 4, 1-15.

Raup, DM (1988) Diversity crises in the geological past. In Biodiversity, ed. EO Wilson and FM Peter, 51- 57. Washington, DC: National Academy Press.

Wilson, EO (1992) The Diversity of Life. New York: WW. Norton & Company

Expert Adds (October, 2013):
The calculations that I used to estimate the upper and lower boundaries on the lifespan of our species were originally proposed by J. Richard Gott III in 1993 (Gott JR (1993) Implications of the Copernican principle for our future prospects. Nature 363:315-319; doi:10.1038/363315a0). After reading this paper when it was first published, I was unable to find it (despite looking) until I read a recent What-If XKCD. Gott's paper is an interesting read and is highly pertinent to the subject of this MadSci answer. Anyone who reads this answer should also read that paper.