How long would it take to fill the earth with humans?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

10 March 2024

Writing from Arusha, Tanzania, Elly Manjale says: “Suppose my miraculous design the world exists in such a way that from, say July 1, 2024 people become immortal. Whoever wakes up on that day doesn't die. On the other hand, current demographic trends as far as births are concerned continue unchanged. How many years will it take before planet earth becomes fully occupied such that it cannot hold any extra soul?”

I will assume that, by “planet earth becomes fully occupied”, Elly means just the land surface (minus water bodies). An average human being standing straight up occupies a space of about 0.25m by 0.25m. That is, one square metre can hold 16 people – 4 x 4 shoulder-to-shoulder, back-to-belly.

Knowing the radius of the earth (about 6,400km) it is easy to calculate the total surface area. It comes to about 129 million square km. This is equal to 129 trillion square metres. Now about 70 per cent of the earth’s surface is covered by water so, that leaves just 30 per cent land, or 39 trillion sq.m

Now, each square metre can take 16 humans, so, about 624 trillion people are needed to completely fill up all the land surface of the earth. How long would that take at the current birth rate?

According to the information website www.worldometers.info, about 24.5 million babies have been born since January this year. And the current world population is almost 8.1 billion. Now, it is tempting to simply divide 24.5M by two to get the monthly births (12.25M) and then divide 624T by 12.25M to get the number of months it would take to fill the planet (51 million months, or 4.2 million years).

But that would be wrong! It assumes that the newly born people will not be giving birth, hence the number of new-borns will remain constant. That’s obviously not true. Therefore, we should first work out the monthly birth rate, that is, number of babies as a proportion of population. Then use that to project into the future.

The monthly birth rate is 0.15 per cent; so, the question now is: if the population grows at this percentage, how many months would it take to move from 8.1 billion to 624 trillion?

That is not a straightforward calculation; it involves evaluation of logarithms. The answer come out to 7,500 months. That is, about 625 years. That’s not a very long time, is it?