What is the evidence for human evolution

by Dr. Ashley S. Hammond

Paleoanthropologists, the scientists who study the human fossil record, use the term “human” for those species that share the general build and appearance of living Homo sapiens (our species) and are inferred to be closely related to us. To some extent, this definition of “human” is synonymous with the genus Homo. Compared to earlier and more primitive hominin genera like Australopithecus, members of genus Homo usually have relatively large brains, relatively longer legs, smaller teeth, smaller chewing muscles, and a more humanlike rate of development as determined from fossilized bones and teeth. In short, species of Homo are bipeds that are smarter and faster than their predecessors and similar to us in many aspects of their diet and growth.

We do not always know whether a fossil species is more closely related to us than it is to earlier species of Australopithecus, which can occasionally lead to differing interpretations about which hominin fossils belong to genus Homo. Keep in mind that the more primitive, also called transitional, members of genus Homo (such as Homo habilis) tend to be the most controversial in their assignment to the genus. Also recall that fossil taxonomy is rooted in the comparative method: We define and classify fossils based on how closely their anatomy resembles or differs from that of other fossils and living species. As we discover additional hominin fossils, our understanding of the anatomy broadens and continually changes our perspective on how these species are related to one another and to us.

Imagine you could travel back in time to meet the first human species. What would you expect these ancestors to look like? Where would you look for them? Would you recognize them as being closer to yourself than Lucy’s species (Australopithecus afarensis)? Homo habilis (1.4–2.4 million years ago, or Mya) is the earliest named species of Homo. You would encounter H. habilis walking around bipedally on open grasslands in Eastern and Southern Africa, but this species would sometimes climb into trees to reach foods. Homo habilis is an early species, so it doesn’t look quite like us yet. It had a small brain (500–800 cm3), only somewhat larger than that of chimpanzees and Australopithecus. With its short arms and legs, its limb proportions look primitive, and if you were to slice through one of its bones, you would notice that the shape of the cross-section looks primitive too. H. habilis was famously nicknamed the first toolmaker (“the original handyman”) based on stone tools found near the first fossil discoveries, but this nickname is chucklesome today, because we now know that stone tool use predates H. habilis fossils.

You might not recognize Homo habilis as a relative based on its body plan and small brain size, but shortly after it flourished a hominin species arose that was built like us: Homo erectus. The most famous H. erectus individual is the Turkana Boy, an adolescent male who was still growing and well on his way to reaching the height of a modern human male. You would have found the Turkana Boy walking long distances in search of meat to hunt or scavenge. A few researchers even think that he was adapted for endurance running on the savannahs.

Although the Turkana Boy was found in northern Kenya, other H. erectus groups ventured out of Africa to occupy locations throughout central Asia and Indonesia. Homo erectus endured longer than any other known hominin species (~2.0 Mya to ~50 kya), and the long chronology and broad ecogeographic range occupied by the species resulted in high levels of variation between the different populations. Even a single population of H. erectus would have displayed levels of morphological variation in the skull that are unmatched in human populations today. Even though Homo erectus individuals looked different from each other, they all shared certain morphologies: a low braincase, a thick and continuous browridge, a sharply angulated back and lower part of the skull, thick bones (including the skull), and a long leg compared to arm length. The cranial capacities of H. erectus range from about 600 to 1,250 cm${}^3$‍, and fossils of this species have been found at sites with a sophisticated hand axe tool industry known as the Acheulean, suggesting more advanced cognitive abilities than in earlier hominins.

Between 700 and 300 kya, we start to see another human species—Homo heidelbergensis—that looks like us. H. heidelbergensis lived in Europe, Africa, and eastern Asia. If you encountered this species in person, you might be confused about what species it is, as it is a hotchpotch of Homo erectus and more modern-looking features. H. heidelbergensis has a thick braincase like H. erectus, but the larger, more rounded braincase of H. heidelbergensis has an average cranial capacity of ~1,200 cm${}^3$‍. It has a projecting face with two large browridges, which stands in comparison to the large, continuous brow in H. erectus. The bones that are known for H. heidelbergensis are large and robustly built—this was one powerful hominin! This is currently the oldest hominin species for which we have mitochondrial DNA (mtDNA) and nuclear DNA, sampled from ~430,000-year-old individuals from the site of Sima de los Huesos, Spain. The DNA reveals an interesting backstory to H. heidelbergensis: It is closely related to Neanderthals based on nuclear DNA, but the mtDNA also documents a relationship with the Denisovan hominins.

Most likely, you have heard of the Neanderthals, or Homo neanderthalensis, as they are known to scientists. You can call these folks the Neandertals or the Neanderthals, but the species was discovered in Neander Valley, Germany—spelled Neandertal in German, so there is a preference for Neandertals nowadays. This species lived in fairly cold or arid locations in Europe and the Middle East 400–40 kya, making it contemporaneous with early members of our own species. Distinctive features of Neandertals (Fig. 7) include a large nose with a large, bony nasal opening on the skull, projecting midface; a bony “bun” at the back of the skull; a football-shaped skull; strong browridges; expanded molar root canals; a curved thigh bone; and a long pubic bone set on a wide pelvis. Brain size in Neandertals (averaging ~1,600 cm${}^3$‍) was even larger than in modern humans! The Neandertals lived tough lives, hunting large game and sustaining injuries on their skeletons that are comparable to the injuries seen in rodeo bull riders! The sequencing of the Neandertal genome has also given us insight into Neandertal mating behaviors. Notably, there are distinctive Neandertal nuclear DNA sequences in modern humans with ancestry outside of Africa, indicating that Neandertals interbred with modern humans soon after Homo sapiens left Africa.

The Denisovans are a population of mysterious hominins known almost exclusively from genomic data. Bone fragments from Denisova Cave, Siberia (dated to 48–30 kya) have been sequenced through mtDNA and nuclear DNA. The morphology of the Denisovans is unknown aside from a partial mandible (Fig. 9) from the Tibetan Plateau (dated to 160 kya) determined to be Denisovan based on preserved proteins on the fossil. Denisovan DNA reveals that they were a distinct species from both Neandertals and modern humans, but they continued to interbreed with Neandertals and modern humans for many years.

What about us? Nearly all genetic studies provide evidence that modern humans originated in Africa, likely in sub-Saharan Africa based on genetic diversity in living human populations. Not surprisingly, the fossil record supports the genomic record: All of the earliest H. sapiens fossils are from Africa! We can identify H. sapiens in the fossil record by their very rounded braincase, vertical forehead, diminutive browridge, a canine fossa, and a pronounced chin (see photo above). The earliest evidence for H. sapiens could be ~315,000-year-old remains from Jebel Irhoud, Morocco, although because these fossils lack a chin and rounded braincase, some scientists hesitate to call them H. sapiens. A skeleton from Omo-Kibish, Ethiopia, dated to 196 kya, is the oldest H. sapiens fossil that is widely accepted by paleoanthropologists as anatomically modern. There is substantial archaeological and artistic evidence that abstract thinking, symbolic behaviors, and advanced tool use accompanied the origins of H. sapiens in Africa.

Although there is a general trend towards increasingly human-like attributes during human evolution, there is always a black sheep (or two) in the family. Several human species (H. floresiensis, H. luzonensis, H. naledi) run contrary to expectations in displaying primitive features, such as small brain and body size, morphologies reminiscent of Australopithecus, or both. Paleoanthropologists are still working to figure out how these relatives fit onto the family tree, but they do not seem to be major players in our species’ evolution.

There is a strong fossil record for human evolution. Members of genus Homo are smart, terrestrially adapted, and ecologically flexible hominins. A currently unknown species of Homo would eventually give rise to our own species in Africa. Before modern humans dispersed out of Africa, Homo species were already living throughout Europe, central Asia, and Indonesia, and our species crossed paths with at least some of them. We know that our species encountered—and interbred with—Neandertal and Denisovan populations, because their genes show up in our genome.