From the first single-celled organism until now? That must be an astronomical number. Whenever I hear the ID folks say that random mutations couldn’t account for the diversity, I just think of all the organisms that have lived in these billions of years and that each one had many DNA replications that provided opportunities for genetic mutation.
I know it’s impossible to determine for sure, so I’d appreciate even wild-assed-guesses.
Bacteria make up by far the largest number of organisms on Earth, to the extent that the number of other organisms is within rounding error. They have the fastest replication time, and have been around longer than other organisms. Therefore other organisms can essentially be ignored in the calculation.
This article estimates the total number of bacteria today at 5 x 10[sup]30[/sup].
Bacteria under good conditions will replicate every 20 minutes. That will be optimal, so let’s say once an hour. That means 24 replications a day, or 8,730 times per year. Let’s call it 10,000, or 10[sup]4[/sup].
The earliest evidence of bacteria goes back 3.5 billion years, but they may not have been so numerous on the early Earth as they are today. Let’s say they have been replicating at the present rate for 3x10[sup]9[/sup] years.
So if my math is right (it probably isn’t) we get something on the order of 1.5x10[sup]44[/sup] DNA replications since the origin of bacteria.
Others are welcome to correct my math. But the number is pretty damn big.
A very rough attempt:
From Growth of Bacterial Populations
Let’s say 4 hours, then.
Also, from Cell (biology) - Wikipedia
So, 3,000,000,000 x 365 x 6 = 6,570,000,000,000 generations.
edit: doesn’t indicate the number of total reproductions, just a very rough number of generations.
I was just going to go with “lots”, but I’ll stick with yours.
Actually, come to think of it…
If you want total number of replications, you need to factor in each and every cell produced in multicellular organisms, and I’ll just wait for Colibri to figure out if that invalidates his approximation or not. However, with the question you seem to be asking, only those giving rise to germline cells are relevant, which would get us back to considering each organism as one replication (within an order of magnitude) (Oh, man, I’m going to get called on that - let’s just say it’s still way less than the number of bacteria).
According to this, the human body contains 10 times more bacterial cells than human cells. Since this is probably true of most other multicellular organisms, it won’t change the order of magnitude of calculations based just on bacterial cells alone.
Curiously enough, this is exactly equivalent to the number of dollars in Uncle Scrooge’s money bin, “five billion quintiplitilion unptuplatillion multuplatillion impossibidillion fantasticatrillion.”
I would note that with 5x10[sup]30[/sup] bacterial cells on Earth, each replicating 24 times a day, genetic events that have a probablity of a trillion-to-one are happening at a frequency of one hundred million trillion times every single day.
A truly profound thought. Thank you.
Wait - does that account for the exponential growth of bacteria? It doesn’t seem like it, but I’m a math idiot.
When I multiply 24 replications by 365 days a year, I get 8,730. But that first replication is 1 into 2 and then 2 into 4 and 4 into 8…at one hour generation times, that’s um…33,554,432 in a day. And each of those bacteria holds the potential for a DNA mutation.
Bacteria in the wild won’t grow exponentially, at least not for long. They’re constrained by nutritional requirements.
The world bacteria population has to be more or less stable since quite some time. If it had been growing exponentially since the first bacteria, well… there would now be more bacteria than atoms in the universe.
Well, sure, but they don’t need to sustain it for long to far outstrip 10,000 in a whole year. That’s how many anthrax spores it takes one person to inhale to get sick.
The numbers are impressive, but the number of bacterial replications is not relevant to the purpose of the OP. Much more useful would be the number of vertebrate or mammalian reproductions. Even that will be astronomical and more than enough to account for evolution.
Back when Lord Kelvin put a top limit of, IIRC, 100,000 years on the age of the earth (since he didn’t know about the heating from radioactivity and was using the cooling rate), this was considered a serious, if not insuperable, problem for evolution. Now that we know that life has been around for nearly 4 billion years, eukaryotes for over a billion and multicellular organisms for at least a half billion, this does not seem an obstacle. On the other hand, if you believe that the earth had been around for only 6000 years, then yes, evolution is impossible.
I just want to point out that when it comes to sexual reproduction, only the sex cells can pass on their genes to offspring. So the probability of a mutant being born is much much smaller than one would calculate based on the total number of DNA replications. That is one of the benefits of sexual reproduction.
Ok, let’s take a stab at vertebrates.
Vertebrates have been around for about 500 million years. Most species probably reproduce for the first time at an age of one year, and after that once a year. Of course, there is a longer time to first reproduction for humans and other large vertebrates, but for most of their evolutionary their ancestors were small, with shorter reproductive times. Let’s take this into account by reducing this to 400 million generations from the origin of vertebrates to the present.
There are at present something like 60,000 species of vertebrates in total. The diversity was certainly lower when they originated, so let’s take the middle value and assume an average of 30,000 species present each year.
Conservatively, let’s assume that each vertebrate species has a global population of 10,000 reproductive females. (Some of course have much larger populations.)
So we get 4x10[sup]8[/sup] x 3x10[sup]4[/sup] x 10[sup]4[/sup], or 1.2x[sup]17[/sup], that is, 120,000,000,000,000,000 or 120 thousand trillion.
PS: This is just reproductive events in which mutations could be passed on, not the total number of DNA replications producing gametes from sex cells, which will be higher by several orders of magnitude.