When Did the First Human Appear on Earth?

It sounds like a natural question, right? My parents are human... and my grandparents are also human... and my grandparents’ parents are human, too... But if we go back in time like this, when does the chain of our ancestors end? If we were created, as for example the Bible says, then of course the chain ends at our creation point. But suppose for a moment that we were not created at a point in time as the Bible tells us, but evolved rather gradually, as science tells us — then what? The chain can’t go backwards forever, because we know there were no human beings, for example, 1 million years ago.(1) So? Doesn’t there have to be a First Human even in the case of evolution, a pair perhaps — an “Adam” and an “Eve” — who became the progenitors of our kind? Doesn’t the biblical story have to be true anyway?

Some readers who have read this other page of mine, on the origins of life, have contacted me asking the above question. In that page I state that there was no first living being, and I try to explain why. Nonetheless, some readers disagree, and argue for the creation of the First Human even under the assumption of evolution, as I just explained in the previous paragraph. They see the conclusion as necessarily following by common sense and pure logic. I would like to show in the present page how it can be logically possible that there was no First Human. The reasoning is simple, but we need an analogy to make it easier to understand.

Let’s think about something related: the evolution of a language instead of the evolution of humans. Let’s use a particular language as an example: the evolution of Greek, from the ancient, to the modern language. (Don’t worry, you don’t need to know a single word of that language to understand this analogy, guaranteed! ) I’ll tell you how Greek evolved from ancient to modern, without ever having a “first modern Greek speaker”. What we’ll learn about Greek is true for every other language that evolved from an older to a later stage (including English, for example), but I prefer Greek because I am more familiar with the evolution facts of that language, and so I will not make false statements. But the point is that, not only it doesn’t matter which language we use as example, but also the analogy carries over to what we are interested at: just as there was no first speaker of a modern language, there was no First Human either. And the analogy is true even in some of its details! Let’s see first what happened to speakers of Greek.

Look at the figure above. Time runs from left to right. The green color is the time when speakers of Greek were speaking the ancient language, and the blue color is for speakers of the modern language. Underneath the colored band I drew a large number of very short vertical lines, each of which is supposed to represent one individual. A single line is one person, and the line to its left is one of this person’s parents, whereas the line to its right is one of this person’s children. There should be many more lines, actually, but there is not enough horizontal space to make the drawing realistic. (Other things are not realistic either, such as the lengths of the ancient and modern stages — but realism is not necessary here.)

Now, let’s see what we have. Each person’s “color” corresponds to the particular version of Greek that the person was speaking. The difference between colors shows how different the versions were. For example, “very green” means “typical ancient Greek”, and “very blue” means “typical modern Greek”. Somewhere in-between there is a region where the language changes. The change is smooth, but also somewhat sudden, if we look at the overall picture. Indeed, since around 300 BC, after Alexander’s death (that’s Alexander III the Great, king of Macedon), until around a couple of centuries after Christ, the change was complete. The historical reasons for the change do not concern us here.(2) What concerns us is the color. Notice this property of the color:

Every parent has a color which is very similar to the color of his/her children. Indeed, from parent to child, the difference seems imperceptible. In language terms, every parent speaks almost the same language like their children. There is some difference, but it is so imperceptible that neither the parent nor the child notice it. (It could be a few words with a different meaning, some new syntactic structure that the child uses, some imperceptible difference in pronouncing a vowel, and so on.) As a result, they can communicate with each other perfectly: the parent understands perfectly the child, and the child understands perfectly the parent.

But now look what happens if we take an ancestor-descendant pair that do not have an immediate parent-child relationship. How much they could understand each other (if they had lived at the same time) depends on how much they differ in color; not on how distant they were in time. For example, we can take two ancient people, both in the very green area on the left, but differing by several centuries from each other — let’s say 800 years. These two people would understand each other, if they could somehow miraculously be brought together. But if we take a pair where the ancestor is from around 300 BC, and the descendant from around 200 AD (making a difference of 500 years), these two people would not be able to communicate (or, in reality, the communication would be extremely limited).

Is there any point in time that we can single out and say, “There! That’s when the first speaker of modern Greek appeared”? Of course not. There is a smooth change, not an abrupt appearance of the first modern Greek speaker. But even though every parent can communicate with their children (and with their grandchildren, clearly), still, the change within those centuries is such that the ancient speaker would not be able to communicate with the modern one, if they could be brought together in time.

Exactly the same idea can be carried over to the notion of change from our ancestor species Homo erectus (that corresponds to “ancient Greek”) to our own species, Homo sapiens (that corresponds to “modern Greek”). Let’s take a look at the following figure.

Is anything different between this figure and the previous? Not really. Only the terms have been changed. And the important points in our analogy are the following:

What was previously language, now corresponds to the DNA of an individual;and the ability to communicate between two individuals corresponds tothe ability to mate and produce fertile children.

So, if we take any two individuals from among “our kind”, Homo sapiens, which is the clear red region, no matter how distant in time, these two individuals would be able to produce fertile children after mating (assuming they were a man and a woman), because their DNA’s would be sufficiently similar. The same thing would happen if we took two individuals of our ancestor species, Homo erectus, from the clear magenta region in the figure. Again, the DNA’s of these two individuals, no matter how distant in time, would be sufficiently similar to allow the birth of fertile children. That’s why we say these two belong to the same species, H. erectus, and those two belong to another species, H. sapiens. But if we take one “purple” and one “red” individual, their DNA’s would be different enough to not allow fertile children to be born. It could be that the chances to succeed in having a healthy child would be nearly zero (say, one in a million). Or, that sometimes children would be born, but those children would be unable to have children of their own, so they would be sterile, “dead ends” as far as propagation of genes is concerned.

Nonetheless, the magenta region does not change sharply to red, but gradually. There is never any “First Red” individual. If we take any two opposite-sex individuals that belong to succesive generations (but not to the same family obviously, avoiding incest), these two individuals (one from the parent’s generation and the other from the child’s generation) would be able to mate and produce fertile children. They would belong to the “same kind”, even if they were taken from the transitional time of around 200,000 to 150,000 years ago, because their DNA’s would be sufficiently close.

How do we know all this? How sure are scientists that the above is true? Have we ever examined the DNA of a H. erectus, and compared it with the DNA of a modern human?

No, no one has retrieved DNA from fossil bones as old as those of H. erectus. The oldest DNA that has been retrieved at the time this text was written (ca. 2005) is a few tens of thousand years old. But paleoanthropologists (scientists who study the origins of the human kind) examine fossils, and are experienced enough to tell with some confidence when a skeleton should belong to one species, and when to another. In reality, we will be nearly 100% certain only when we obtain the DNA of a H. erectus individual, and this will take time, because the older the fossil the more improbable it is that the DNA has been preserved somewhere. But it is not impossible to find it, somewhere on this planet. Perhaps deep frozen under the vast unexplored ice sheets of Siberia (if H. erectus ever reached there).

But suppose that we find such a DNA molecule, and after we compare it with a modern DNA molecule we realize — to our surprise — that in fact the two DNA’s are similar enough to qualify as “same species”: the two individuals would in principle be able to have fertile children after mating. Would this discovery demolish the explanation presented in the previous paragraphs, about the nonexistence of a “First Human”?

Not at all. Such a discovery would simply push the species-distinction era further back in time. All right, so it would not be what we call H. erectus the species of which we would have the honor to be the descendants. We would be the same species with them, in that case. So? There would be another species, further back in time, that would have sufficiently different DNA to disallow mating and production of offspring with “us”. But still the change between that species and the erectus-sapiens species would be gradual-and-yet-abrupt, as the figure with the magenta and red colors depicts it. And so, still there would be no “First Human”.

This notion of “gradual-and-yet-abrupt” change, by the way, is a hard one to grasp, because the words sound self-contradictory: is it gradual, or is it abrupt? But I think the colored figures, above, depict in a nice way how the change can be both gradual and abrupt, without any contradiction:

The change is gradual because between any two successive generations the difference (in DNA, in language structure) is minimal; so, successive generations can “communicate” (pass on their genes, mutually understand their ideas). Thus, the change is gradual when we take the magnifying glass and look at it from up close.
But the change is also abrupt when viewed at a larger scale: for a very long time, there is almost no change at all (same species, same language), which is called stasis in biology; but within a very short period of time (“short” relatively speaking), changes occur and accumulate “fast”, and we arrive at a different “kind” (another species, another language). The change is abrupt when we zoom out and look at the overall picture from afar, taking a bird-eye view.