Charles Darwin was a polite man.

But when it came to the subject of barnacles, he once burst his collar: “That damn variation” he scolded in a letter to the botanist JD Hooker in 1850.

At that time, Darwin was working on a monumental work on marine animals, of which barnacles are one of the best-known representatives.

While the first specimens he studied were diverse enough to be easily classified taxonomically—that is, assigned to one species or another—as the body of material grew, things became more and more confused.

Ulf von Rauchhaupt

Editor in the “Science” section of the Frankfurter Allgemeine Sunday newspaper.

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Now there were variations within individual barnacle species, which sometimes seemed to merge seamlessly into the differences between the species.

How do you decide whether two specimens still belong to one species or not?

What is a species anyway?

The question was still open when Darwin's major work On the Origin of Species was published nine years later, in which he remarked resignedly: "Until now no definition has been able to satisfy all naturalists.

And yet everyone has a vague idea of ​​what they mean when they speak of a species.”

What is a species?

The only problem is that not everyone is necessarily talking about the same thing.

In fact, more than two dozen different species terms are in use today.

The best known was defined in 1942 by the ornithologist and evolutionary biologist Ernst Mayr, according to which living beings belong to a species if they can be crossed and produce offspring capable of reproduction.

The crux of the matter is that for many organisms - such as barnacles - it would take a great deal of effort and patience to determine which species belong to, and for those that do not reproduce sexually, such as bacteria, not at all.

And of course not with extinct creatures.

If the remains are too old to still contain genetic material, one has to rely entirely on characteristics of the external shape, i.e. their morphology, when determining the species.

On the other hand, if they are young enough for the fossil record to resolve evolutionary changes over time, it becomes doubly difficult to distinguish such morphological species from one another.

New discoveries exacerbate the problem

This is exactly the situation with the species that gave rise to our own.

"Paleoanthropology is in exactly the same position today as Darwin was with his barnacles," says Francis Thackeray of the University of the Witwatersrand in Johannesburg.

When the first hominid was discovered in Africa in 1924, Thackeray says it was easy to describe it as a new species, Australopithecus africanus.

Today, however, researchers assign several thousand fossil finds to 23 hominid species.

The most recent are three spectacular South African finds: First, Australopithecus sediba, published in 2011.

Second, “Little Foot”, the most complete skeleton yet of an early hominid, whose publication as Australopithecus prometheus is imminent.

And thirdly, Homo naledi, the largest with 15 individuals so far,