Scientists assert that all life evolved from a single common ancestor. The implications of this are huge. Either this is true, and all life on earth is related, or each living species is entirely independent of each other. In the former case we could draw the relationships of every animal on earth in a tree diagram, also known as a phylogeny.
Here the tips represent all the species on earth and the branches represent their ancestors.
In the latter case (science is incorrect and evolution isn’t real) we would represent species on earth as a series of points, unconnected and unrelated.
This is just a simple visual illustration, but the implications go much further than that. Our understanding of evolution informs how we make medicine, control pest animals, rear domestic species, and predict disease dynamics. Maybe most importantly, it dictates the story we believe about our origins.
It’s a big deal. But could it have even happened?
Creationists assert that the human genome is so large and complex that it could not have arisen simply by random mutation. It would just take too long and so it couldn’t have happened in the 4 billion years that life has been on our planet.
Is this true? We can use evidence and a little bit of arithmetic to check the validity of this argument.
The question we want to answer is…
What is the minimum amount of time it could have taken to evolve a human?
This perfect animal specimen is encoded by DNA approximately 3221.49 Mega-bases long. That is 3,221,490,000 individual nucleotides (commonly referred to as A’s, T’s, G’s, and C’s).
Ok, break for biology lesson. DNA is the stuff that codes for who you are. What you look like, the order in which your parts develop, the shape of your eyes, the curvature of your toes, your skin color. It’s all determined by your DNA. In many cases, the specific sequence of your DNA leads to changes in development. The difference between having DNA that looks like AAATAAA or AAAGAAA could literally determine if you live or die . All organisms have DNA and it works the same way for them as it does for us. All the similarities and differences between a human and a sea slug are determined by similarities and differences in our DNA. Bio lesson over.
A rough estimate of the mutation rate for humans is 45 total substitutions per generation. Admittedly, this is very high. So let’s just be conservative for the sake of being fair and say that the mutation rate is 1 mutation per generation.
Starting from a random template of 3.2 billion nucleotides it would take AT LEAST 3.2 billion generations to make a human through random mutation only. If each human generation is 30 years that would take 96 billion years to make a human!
If that was the minimum time it took for a human to evolve, then the creationists are correct. However, the way we are calculating this is very oversimplified. Let’s add some things to make this more realistic.
Another quick bio lesson. Although DNA codes for who you are, that’s not exactly what your building blocks are. What makes you you are proteins. Proteins are made up of amino acids, which are coded for by DNA. For example, DNA with the sequence CGT codes for the amino acid Arginine. There is a lot of redundancy in the genetic code. This means multiple DNA sequences code for the same amino acid. So, CGT, CGC, CGA and CGG all code for Arginine. (the genetic code) In this example you will notice that the first two letters are the same but the third letter doesn’t matter. Anytime the third position mutates it does not result in changes in protein sequence. This means that we can say, on average, 1/3 of our DNA sequences doesn’t really matter. As we will see in a second, this is a critical point. But there’s something else we need to consider: introns. Intron is a silly name referring to a region of DNA that doesn’t code for proteins. Their sequences are essentially random and don’t really matter. They do some important things but for our purposes let’s just pretend they are entirely random DNA sequences. Back to our calculations.
Out of all 3.2 billion nucleotides we only have about 50,000 genes. On average, these genes are 15,000 nucleotides long. That means the coding genome size is only about 750 million (not 3.2 billion). But only 2/3 of these nucleotides really matter. Now we will start with a more realistic random template of 500 million nucleotides.
We can do the same calculation as before with a starting number of 500 million but instead let’s consider something we didn’t the first time. Out of those 500 million random nucleotides, ¼ are already the correct “human” nucleotides, because there are only 4 possibilities (A, T, C or G). One out of every 4 random ones will be right from the start! Now, we’re down to 375 million nucleotides.
At our rate of 1 mutation per generation it would take 375 million generations or ~11.25 billion years (at least!) to make Mr. Goldblum. This is 9 times less than our previous estimation but still more than the entire time that earth has existed.
If it takes at least 11.25 billion years to evolve a human by chance then it definitely wouldn’t happen in 4 billion years if the process was guided purely by random mutations. If this was the whole truth then, once again, the creationists would be right. But guess what? It’s not the whole truth.
There’s another thing we need to take into consideration. That is that there is a small possibility that entire genes can be duplicated all at once (in fact, entire genomes can be duplicated all at once but let’s ignore this one). For a given gene, it may undergo a duplication about once every million years.
Let’s factor that into our calculation. Start with 375 million random nucleotides. Let’s say it takes at least 450,000 years to randomly mutate one gene from a random starting sequence of DNA (30 yr generation time*15,000 nucleotides per gene). In 1 million years we have 2 genes by random mutation and 2 more genes as a result of whole gene duplications. Let’s assume this is a constant rate of 4 genes (or 60,000 nucleotides) per 1 million years (this would be exponentially increasing as I’ve presented thus-far but in reality entire genes can be lost also, so it’s better if we say it’s just a constant rate). That halves the minimum time necessary to achieve a human genome (~6 billion years).
Once you remember that the mutation rate we are using (1 mutation per generation) is very conservative (3-9 mut/gen or up to 45 mut/gen) the time it would take a human to evolve from scratch seems a lot more reasonable. Add in a shorter generation time (a human could definitely make a baby in 20 years instead of 30) and it could take 1.3 billion years or even only 89 million years.
This is still a massive oversimplification of both the biology involved and the math needed to explain it. We could also consider: rates of mutation loss, rates of gene loss, rates of whole chromosome or genome duplication, or rates of DNA insertions/deletions. We could do the calculation using different organisms that have smaller or larger genomes. There’s a panoply of other ways we could do this.
The main point is that we have fulfilled the requirements of our original question using real biological information and…
…it is absolutely not impossible for humans to have evolved in the time we have been on earth