How much evidence have scientists found for human evolution?

There are so many fossils that nobody knows the full number. And that’s just the beginning.

I don’t know how many hominin fossils there are in the world. There is no tabulation. The pace of discovery now is too fast to track. Each year for the last decade, anthropologists have unearthed hundreds of fossil specimens from extinct hominin species and populations.

Fossil material from Sima de los Huesos with members of the team. Photo: Javier Trueba/Madrid Scientific Films

By 2012, the Sima de los Huesos hominin assemblage, near Burgos, Spain, numbered more than 6500 specimens from at least 28 individuals. Many more fossils are recovered in every field season. In South Africa, the Rising Star hominin sample today numbers more than 2000 specimens from at least 18 individuals. This deposit of hominin fossils was completely unknown until 2013. From just two caves, that is nearly 9000 fossil hominin specimens.

There are many other sites with hundreds of specimens each. One of my favorites is Krapina, Croatia, which preserved the remains of more than 30 Neanderthals who lived some 120,000 years ago. Hadar, Ethiopia, is a large area of sediments deposited by ancient rivers and lakes between 2.3 and 3.6 million years ago. Paleoanthropologists there have unearthed many hundreds of hominin fossil specimens over the last 45 years, including the famous “Lucy” skeleton. Sterkfontein is a South African cave system with several deposits containing fossil hominins, along with other creatures. To date, more than 600 specimens have been found from the Sterkfontein caves. The ancient fossil deposits around Lake Turkana, Kenya, including areas known as Koobi Fora, Ileret, Nariokotome, Lomekwi, and Kanapoi, have likewise generated hundreds of fossil hominin specimens.

Fossil hominin material from Krapina, Croatia. Pictured is my friend, Jakov Radovčić.

The National Museum of Natural History reports that the hominin fossil record includes the remains of more than 6000 individuals. Counting only modern human skeletal remains from before 10,000 years ago, I think that’s conservatively low.

Of course, the total number of fossils depends on how we count. Partial skeletons make that challenge especially clear. Olduvai Gorge fossil hominins have been listed by individual, up to “Olduvai Hominid 82” and beyond. Some of those individuals are known only from lone teeth. Others are partial skeletons made up of scores of fragments, like OH 62.

Some sites, like the fossil localities in the Omo Shungura region of southern Ethiopia, present fragments of a large number of hominin individuals who lived over many thousands of years, with no partial skeletons. Nearly every tiny fossil or handful of teeth is another ancient individual. We may not know much about each one, but we can see something about how they vary.

MH2 and MH1 fossil skeletons from Malapa, South Africa. Photo credit: Lee Berger.

Other sites are known for their partial skeletons. Malapa has two, with more individuals represented by bones still in breccia. The great Neanderthal sites of France, Belgium, Spain, Italy, and Germany: La Ferrassie, La Chapelle-aux-Saints, Spy, Le Moustier, Sima de las Palomas, Altamura, Feldhofer Cave, Monte Circeo. The sites of the Near East: Skhūl, Tabun, Kebara, Amud, Qafzeh, Dederiyeh, Shanidar. The list goes on and on, each site with partial skeletons of ancient individuals from populations that no longer exist. Skeletons are essential to understand how different parts of the body were subject to different adaptive and nonadaptive histories. When we look to periods older than a million years ago do fossil sites with skeletons become much more exceptional, but there remain a few: Malapa, Nariokotome, Aramis, Dmanisi, Sterkfontein, Hadar, Woranso-Mille, Olduvai Gorge, and Koobi Fora. Their importance is evident, no matter how we count the pieces.

Anthropologists have long had a skull fixation. There are more than 200 relatively complete skulls from hominins other than modern humans. Some sites, like Sangiran and Ngandong in Indonesia, and Zhoukoudian, China, are mainly known for their series of skulls, numbering close to a dozen for each of these, although other bones of the skeleton have also been found in these places. One partial skeleton is associated with the remains of one of the earliest modern human skulls in Africa, for others from sites like Jebel Irhoud, Morocco, Laetoli, Tanzania, and Herto, Ethiopia, the skulls have been the main focus.

Hominin skull casts on exhibit at the National Museum of Natural History. Photo credit: Lenny Flank (CC)

Once you realize how much evidence of evolution comes from modern humans, there’s almost no end to it. Research collections of modern human skeletal remains number hundreds of thousands worldwide. Every roadcut and building foundation has a chance of running into ancient human remains. When they do, many of these archaeological remains are studied by scientists and preserved in collections.

Thousands of skulls are housed in the Naturhistorisches Museum, Vienna. Photo credit: John Hawks.

The great natural history museums of the world, many founded in the late nineteenth century, had a mission of documenting evidence of human variation around the world. In those days, expeditions and excavations collected tens of thousands of contemporary skulls from around the world. Charles Darwin only touched one Neanderthal fossil, but he and other anthropologists of his time still knew an enormous amount about the variation of humans around the world, enabling Darwin to form some hypotheses about human evolution that we still accept today, and others that fossil evidence has rejected. Many of those enormous collections of human bones still exist, and today scientists like me study them to document the recent evolution of human populations. I’ve studied thousands in my work.

The changes that we see in the skeletons of populations of the last 10,000 years were some of the oldest scientific findings of anthropology. Those results have been rejuvenated by the study of ancient DNA recovered from the bones, which has confirmed some very rapid evolutionary changes in that relatively short time.

Denisova 3 finger bone (cast). Photo credit: Thilo Parg.

Ancient DNA has transformed the quantity of evidence from some fossil sites. One site with extremely few and fragmentary skeletal remains, Denisova Cave in Russia, preserves ancient DNA with a higher fidelity than any other site from such ancient times, stretching from more than 30,000 back to 100,000 years ago. In 2014, Matthias Meyer and colleagues recovered DNA data from a tiny fragment of finger bone known as Denisova 3, amounting to the whole genome sequenced some 60 times over. Counting such evidence is like counting the leaves of a million trees.

Transformative discoveries from DNA show that the evidence for our evolution is not only from looking at fossils, it requires us to look within and beyond the fossils. Scientists now are looking not only at DNA evidence but also at ancient proteins, epigenetics, and other biochemical traces. To test hypotheses with such evidence, we must know just as much or more about the biology of living humans and our living primate relatives. Today some questions about ancient human relatives are not limited by the fossils, they are limited by how little we know about the workings of human biology.

Homo naledi bones from the Dinaledi Chamber, South Africa. Photo credit: John Hawks.

So you see, there’s no simple answer as to how much evidence of human evolution we have found. There’s an awful lot, in some ways more than we can possibly count. The last time scientists tried to generate a compendium listing basic information about fossil remains from the important sites, it stretched to four volumes covering more than 1800 pages. That was more than ten years ago.

Some anthropologists say that we don’t need more evidence, we need better ways to use the evidence we have found. I say if that were true, then the fossil discoveries of the last decade should have given us more of the same things we already had seen.

Instead, scientific exploration keeps generating truly unexpected discoveries. We have found new ancient populations, both with new fossil discoveries and from genomes. We have found tendrils of our genetic ancestry in unexpected places, and fossil hominins with forms that we never imagined. The past is not the comfortable world that I grew up with in Time-Life books from the 1970s. It is strange and new, and changing.

We must keep exploring for more.