Gradual Evolution
Q: If evolution is a gradual process, why is there no fossil evidence of all intermediate stages of human evolution?
Hello, Sam Lee, and welcome to Quora!
You already answered your own question, believe it or not. It’s precisely because “evolution is a gradual process” that we will never find fossil evidence of “all intermediate stages” of human evolution. Not that we haven’t already found hundreds of so-called “transitional” fossils between modern humans and their distant non-human ancestors, mind you, but the very gradual nature of how evolution works means that there are not any defined “intermediate stages.”
People talk about looking for the “missing link” as if evolution occurred like links on a chain, with one link clearly different than the next, or like rungs on a ladder, with each rung a distinct “stage”. But that’s only because they don’t appreciate what you have already realized — evolution is a gradual process and not one that involves one distinct species suddenly turning into another distinct species.
Here, perhaps this will help:
Evolution of a species over time is a gradual* process, which can be represented by the following color gradient from blue to yellow:
The pixels on the far left side of the image above are clearly “non-yellow.” In fact, they are distinctly blue. The pixels on the right side of the image, on the other hand, are clearly “yellow.” And it is clear that the pixels gradually transition from blue to yellow (or “not-yellow” to “yellow”) as you move from left to right. However, despite the fact that the pixels are clearly transitioning from blue to yellow as you move from left to right, if you look any individual pixel in the image above you will never find one that doesn’t appear to be the exact same color as the pixel to the left and to the right of it. There is a difference, but it’s just too slight to notice with the naked eye. And there is also no pixel you can point to that is definitively the “first” yellow pixel as you move from left to right.
Here, let me show some extreme close-ups that display the actual pixels from some randomly selected spots:
In each example, all the pixels look totally the same, don’t they? And yet, when you look at the original image as a whole, you know that the pixels on the far left are completely different from those on the far right. At no point is any pixel next to a pixel that is a different color (at least not to the extent that we can observe with the naked eye). It is possible to detect a subtle difference between neighboring pixels using sensitive scientific instruments (in this case, the Eyedropper tool in Adobe® Photoshop®):
But, as mentioned before, these differences cannot be detected with the naked eye and the two adjoining pixels are, for all intents and purposes, exactly the same color.
It’s the same with evolution. If you go far enough back in time, you will find our distant ancestors who are clearly “not-human.” But as you travel forward in time you will never find a moment when the first “non-human” gave birth to a “human” since the individual changes were too small to notice even though they eventually combined to become a large change.
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So, what about the so-called “missing links” that evolution deniers love to talk about? Simple — there aren’t any! As shown above, evolution itself is a gradual process usually involving very small changes that accumulate over very long periods of time. Fossilization, however, is a relatively infrequent occurrence and we will never be able to find examples in the fossil record of every single generation between a distant ancestor belonging to one species and a later descendant belonging to a different species. So, while the evolutionary process from one species to another may actually be this:
At best we may only be able to find fossils for the following individuals within that evolutionary line:
Some people see these fossils and assume that each represents a member of a distinct rung on the ladder or link on the chain between point A and point Z. But they really are just snapshots in time of a gradual process. Scientists use a variety of methods, including comparative anatomy, comparative location within geological strata, radiometric dating, etc., to figure out which fossils belong on the same evolutionary timeline and where on the time line they fit. And sometimes they make mistakes and have to make adjustments. But the fact remains that every single fossil ever discovered represents a transition between the previous generation and the following generation.
Regardless of this fact, however, evolution deniers will claim that this is “flawed” because there are obviously gaps or “missing links” in the gradient. OK, so over time we discover more fossils and are able to fit them into those gaps:
Satisfied?
Of course not! In fact, now there are even more gaps than before!
Seriously? When will this end? How about now:
Nope, even more gaps.
How about now:
Don’t be silly! Just look at all those gaps you have there! In fact, now you have 10 times as many gaps as you did before!
*SIGH*
In fact, the only way to have no gaps at all (no “missing links”) would be to find a fossilized example of every single generation that existed from the starting point to the end point:
All of which is to say that the problem isn’t “missing links” at all, but how well we can fit the “existing links” into the gradient. And we can do that by evaluating the structure, the location, the prominent features, etc., of each individual fossil. There are no “missing links” because each and every generation is a link between the generation before it and the generation after it and we don’t need to see every single generation to plot the course of evolution. Anybody who demands to see the “missing links” or “transitional fossils” is really just not getting the point.
* The gradients I created for this post were meant to illustrate the type of gradualism first proposed by Charles Darwin and still accepted by the majority of evolutionary biologists today. I have occasionally been asked, however, to create a gradient that would illustrate the concept of “punctuated equilibrium” that holds that species remain in a state of relative “stasis” for many generations before undergoing relatively rapid speciation when faced with significant changes to their environments. So here is my best attempt:
The difference in length between the periods of “stasis” and the periods of relatively rapid speciation (the “punctuations”) in this gradient is purely arbitrary on my part, but hopefully it illustrates that even during periods of relatively rapid speciation you will still never find any pixel that is noticeably different from the pixel to the left or right of it. For example, an enlarged view of the punctuation period between the blue and purple stasis periods showing the individual pixels would look as follows:
And keep in mind that (a) punctuated equilibrium is not universally accepted and (b), although there exist some debate over how long the periods of rapid speciation last, supporters of punctuated equilibrium generally place the figure between 50,000 and 100,000 years.[1] In other words, still gradual, but just not quite as gradual. No matter how you look at it, members of one species just don’t give birth to members of a different species.
Futurama Evolution Debate
https://youtu.be/yX6p5UwFhh4
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