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Understanding Evolution - How Humans and Apes Fit Into the Tree of Life

I came across a question on Quora the other day that seemed to reflect a common incomplete understanding of evolution, If it took 5 million years for today´s humans to evolve from the apes, how long time did it take for today´s apes to evolve from their origin?. There are a few issues with that question, but rather than enumerate them all here, I'll just jump into the explanation, which will hopefully make it clear as we go. The one thing I'll say up front is that we diverged from chimps & bonobos more like 6 million years ago, not 5 million.

It all depends on what perspective you want to take, and which starting point you want to go with. When people bring up the 6 million years for humans to evolve from apes, what does that really mean? Take a look at this diagram:

Hominid Evolutionary Tree
Click to Embiggen
Image Source: The Open University - Studying mammals: Food for thought


That's one probable evolutionary tree for us over that time (the exact details are subject to debate). Notice how bushy it appears. Populations kept on splitting and splitting and splitting, and most of those species ended up going extinct. We're the only surviving members of that lineage (though Neanderthals nearly made it to the present day). But, if you wanted to ask, how long did it take for humans to evolve, where would you pick as your starting point in that diagram? It just happens to start with Orrorin tugenensis, but that's only because that's where that artist decided to start it. They could just as easily have started with Ardipithecus ramidus, and you could say it took us 4 million years to evolve from that. Or, they could have skipped ahead and started at Homo habilis, and you could say that it took us 2 million years to evolve from that. Or, you can notice that Australopithecus boisei and us are pretty distant cousins on that tree. If A. boisei had managed to not go extinct, or to have left descendants that kept on evolving into some new species, there might be another ape alive right now more closely related to us than chimps and bonobos. So, then we might be saying that it took us 3 million years to evolve from apes. But it wouldn't be anything different about how we evolved - it would just be the fact that we had a still living closer cousin to compare ourselves to. (Note that that terminology is a bit misleading, as you'll hopefully understand after reading this full entry - we are simply apes ourselves.)

Here's another diagram, this time including the still surviving great apes, but not showing all the ancestors or extinct species from side branches that died out:

Ape Evolutionary Tree
Click to Embiggen
Image Source: BOSCOH.com - Milestones of Human Evolution from Paleontology & Bioinformatics


That's where the 6 million year number comes from. It means that 6 million years ago, there was a population of animals whose descendants would eventually become chimps, bonobos, and humans. It was the last common ancestor of us three surviving species. It took each of our species 6 million years to evolve from that population. But recall the branching pattern from the previous diagram. It wasn't a straight line from that population to each of us species that's still around. It split and split and split in a bushy pattern. In the lineage that led to us, only one species survived to the present - us. In the lineage that led to chimps and bonobos, those two species survived to today.

And you don't have to pick just chimps and bonobos. If you look at gorillas, our common ancestor with them was alive roughly 8 million years ago. So, it took 8 million years for gorillas to evolve from that ancestor. It took chimps 8 million years to evolve from that ancestor. It took bonobos 8 million years to evolve from that ancestor. And it took us humans 8 million years to evolve from that ancestor. Chimps, bonobos, and us share a common portion of that 8 million years. Chimps and bonobos alone share an even longer common portion. It would be similar to asking, how many generations did it take to get from your great-grandparents to you, or to your brother, or to your cousin, or to your second-cousin? In all cases, it would be three generations. For you and your brother, you'd share most of that lineage, starting with your great-grandparent, then your grandparents, and then your parents. With your cousin, you would only share your great-grandparents and grandparents. And with your second cousin, it would only be your great-grandparents. There are a lot more greats than that considering our evolutionary history, but it's the same concept. We share more of our lineage with chimps and bonobos than with gorillas. And we share more with gorillas than with orangutans. And we share more with orangutans than with non-apes.

If you want to go further and ask how long it took for apes to evolve, it really depends on how far back you want to go. Here's another diagram:

Primate Evolutionary Tree
Click to Embiggen
Image Source: ResearchGate


Now, we get into a problem of semantics. In language, apes have a name to describe them as distinct from monkeys. But we're not really a completely distinct group. To have a distinct group in classifying these types of things, all members of that group should share a common ancestor that no other group can claim in its ancestry. Apes have such an ancestor around 20 million years ago. The only descendants of that specific animal are apes. But monkeys don't have that type of unique common ancestor. There's no single ancestor of 'monkeys' that isn't also an ancestor of apes. We're not two separate groups. Us apes are really just a specialized subset of monkeys without tails. But, if your question is just when 'apes' first appeared, then like I already said, the last common ancestor of all apes was alive around 20 million years ago.

But why stop there? When biologists say that all life on earth is related, they mean it. All life on earth shares a common ancestor. If you go back far enough, you can find our last common ancestor with chipmunks (~90 million years ago), or with a triceratops (~320 million years ago), or with a goldfish (~432 million years ago), or with an apple tree (~1.6 billion years ago), or even with the streptococcus bacteria that may have given you your last sore throat (~4.3 billion years ago). So, if you want to start at the beginning, you have to figure out when our earliest, earliest single celled ancestors were alive. The problem is that it's hard to find evidence of things that nearly inconceivably ancient, but it was probably more than 4 billion years ago. So, in that sense, it's taken humans over 4 billion years to evolve. It's take starfish over 4 billion years to evolve. It's taken e. coli over 4 billion years to evolve. It's taken oak trees over 4 billion years to evolve. Etc. Etc. Every organism alive is the end result of all that evolution leading up to where it is now.

Complete Evolutionary Tree
Click to Embiggen
Image Source: evogeneao Tree of Life


So to summarize, it's taken chimps, humans, and bonobos roughly 6 million years to evolve from our last common ancestor. It's taken all of us apes as a whole roughly 20 million years to evolve from our last common ancestor. You can keep going back in our ancestry until somewhere more than 4 billion years ago to the first life, that was the ancestor of everything alive today.

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There are some really good trees of life and similar type pages to play around with. Here are a few (I already linked to one above, but it's worth repeating). They mostly include only the tips of the tree for organisms that are still alive. So, you won't necessarily be able to find an Australopithecus or a Tyrannosaurus, but even just sticking to living animals, it's a huge, huge tree.

 

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