# If Evolution is True, How Many People Should Be on Earth?

I was emailed about a fairly “out-there” claim by YECs.

Evolutionists argue that humans (i.e., the genus homo ) have been on the Earth for roughly two to three million years. Using statistics, one can arrive at an estimate for how many people would be predicted to be on the Earth at different points in history. For example, accounting for factors such as war, disease, and famine, and assuming humans have been on the planet for only one million, rather than two to three million, years, we find that there should be 10^2,000 people on the planet today.14 There are, however, not even 10^10 people on the Earth. In fact, if three-feet-tall humans with narrow shoulders were squeezed into the Universe like sardines, only 10^82 people could fit into the entire Universe. It would take 10^1,918 (minus one) other Universes like ours to house that many humans.

Footnote 1 is a reference to this paper, which gives the derivation:

There are many ways to show where the math goes wrong here, and it’s hard to know where to start. What would be the most clear and direct way to show the mistake?

I also want to know if this is claim adopted by other YECs. Seems that it might be idiosyncratic to this guy.

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Ask how many people there were to build the pyramids, or take part in the exodus.

If we apply the same reasoning to houseflies, how many should we have today if the earth is 6000 years old?

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I’ve seen variants of it before. Veterans of the RationalSkepticism forum (@Rumraket?) might remember a dentist named Steve, IIRC, who spent a lot of time defending this line of argument.

Incidentally, the second article is fascinating. First, score one for the Salem Hypothesis. Second, check this:

And he goes on to support this interesting viewpoint with citations of real scientific publications on the origins of sex.

I have seen this claim before from various sources. It seems that some YEC believe that men and women are different species.

Granted if that were true, it would explain a lot!

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My favorite is to accept the argument and work with it. Presume a six thousand year old earth. Let’s start with rabbits. Bacteria would work.

Once it is recognized that carrying capacity is a thing, the discussion turns on its head. Given that for every population which is at least stable, more offspring are produced than will successfully secure resources, it is evident that the most competitive will, on average, succeed. What are the implications of that?

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Here’s a variant on that:

How long would it take a dividing colony of bacteria to consume the planet?

This is a nuanced question, and requires that we slowly build up our understanding of bacterial metabolism and growth. The key to this question is in the phrase dividing colony, which means that we begin with one bacterium and allow that cell to divide for days, or weeks, with infinite nutritional resources. If we continue to feed the growing colony, how long until the earth is exhausted of carbon-based resources?

Exponential growth of bacterial cells, dividing every 33.3 minutes, for 8 hours. By Niko McCarty, made with Python / Seaborn.

Well, let’s start by estimating how quickly bacterial cells divide. Using Python and Matplotlib, I’ve made a line plot that shows the exponential growth of a bacterial colony (using numerical integration with the forward Euler method), assuming a division time of 33.3 minutes.

After just 8 hours, a single bacterial cell will give rise to more than 1.5 million cells. If we let the colony grow for several days (with infinite nutritional resources), their numbers quickly surpass the most absurd of values. After a week, our theoretical colony would surpass 10¹¹⁹ cells, a number so vast as to be incomprehensible. This is nearly one hundred orders of magnitude more than the estimated number of stars in the universe.) (10²⁴).

For comparison, the mass of the known universe is a paltry 10⁵⁶ grams. If each E. coli weighs one picogram, then 10⁶⁸ E. coli would weigh as much as the known universe…a feat that would (theoretically) take a few days.

Exponential growth of bacterial cells, dividing every 33.3 minutes, for a week. The magnitude of cells soon exceeds all reasonable explanation for our rudimentary thinking. By Niko McCarty, made with Python / Seaborn.

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These arguments seem to be related to an argument I came up with years ago, when people were coming up with scary projections of how many humans there would be in the future. Mine was an attempt to make the ultimate scary projection. If the human population grew at 2% per year, then ultimately it would cover the earth and there would come to be a ball of people with the earth in the middle. When that ball had a radius of 50 light years, it would be expanding at the speed of light. Of course this was leaving out where enough food or air would come from. Making a rough estimate of the volume of an average human one could figure out how many people there would be in the ball and from that how many years it would take until we had that many. I think it was about 5,000 years, but I may misremember that, and don’t have the energy to do the calculation again.

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I knew it. Relativity is wrong!

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Indeed. I couldn’t help but check author Jeff Miller’s background: Ph.D. Biomechanical Engineering, Auburn University

Dr. Miller’s article was certainly “entertaining” in its use of numbers—but I was particularly struck by this gem:

Let us further assign a reasonable estimate of a “generation” to be 38 years. This means that each couple has had all of their children by age 38.

Ignoring for the moment that a human generation has typically been defined somewhere between twenty and thirty years because that is how long it takes for a child to grow up and form a new family—depending upon the expectations of the culture—why does Miller think that human reproduction ceases around 38 years old?

Of course, there are far more serious numerical fallacies in the article but this particular nugget struck me as the author spitballing without much serious investigation to see what population statisticians and anthropologists have to say on the topic.

Worse than that. It’s not a proper definition of “generation” regardless of the numbers. A generation length is the mean age of a parent at the birth of a child, averaged over all children. (Though it may be limited to those children who survive to adulthood or who themselves reproduce.)

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Just a reminder that Office Hours threads are different.