Human evolution followed a single straight ladder in which one species replaced the next.

The familiar “march of progress” image shows human evolution as a single file: one ape-like form gradually becoming more upright and human until it ends with Homo sapiens.

Evolutionary theory itself did not require such a ladder, but the early human fossil record was sparse enough that a few famous specimens could be arranged that way in textbooks and museum displays.

As more fossils were discovered, the neat sequence fell apart.

Different hominin species often overlapped in time, sometimes for hundreds of thousands of years. Some had large brains but retained other older traits; others combined anatomy in different ways. Many branches clearly ended without becoming direct ancestors of modern humans.

“Hominin” refers to humans and the extinct species more closely related to us than to chimpanzees. Their history looks much more like a branching bush than a ladder.

Genetics made the picture even less linear. Neanderthals, Denisovans, and some populations of Homo sapiens interbred, so branches that had separated could sometimes exchange genes again.

That does not mean the relationships are unknowable or that one species never gave rise to another. The correction is to the single-file progression: human evolution contained many contemporaneous branches, side lineages, extinctions, and some interbreeding.