- Describe the geographic range, age, anatomy, and behavior associated with Neanderthals.
- Explain how genetic evidence reveals interbreeding among Homo sapiens, Neanderthals, and Denisovans.
- Identify inherited Neanderthal and Denisovan genes and connect them with their effects.
- Summarize the leading explanations for the disappearance of other hominins.
- Explain how adaptive introgression may have supported the success of Homo sapiens.
Neanderthals and Modern Humans
Neanderthals lived widely across Eurasia, repeatedly interbred with Homo sapiens, and left genetic variants that still affect some people.
Homo sapiens is now the planet’s only surviving hominin species, but several hominin groups lived in Eurasia about 100,000 years ago. Genetic evidence shows that our ancestors encountered and interbred with some of them, leaving traces that help anthropologists reconstruct human history.
Where and When Neanderthals Lived
Scientists have studied Neanderthal fossils since the 1800s. Finds extend from Portugal and the United Kingdom across Europe to Siberia’s Altai Mountains, with additional remains as far south as Israel.
- The earliest Neanderthal-like remains are about 450,000 years old. They come from Sima de los Huesos, or “Pit of the Bones,” in northern Spain.
- The latest known Neanderthal fossils date to roughly 40,000 years ago and occur at several western European sites.
Bodies, Skills, and Social Care
Neanderthal anatomy closely resembled ours, although several traits differed from those of Homo sapiens.
- They were comparatively short and stocky, with sturdy limbs.
- Their brains were large.
- Prominent brow ridges and large noses characterized their faces.
- Their braincases were more oval than the rounded braincases of Homo sapiens.
Genetic Evidence of Interbreeding
In 2010, researchers first sequenced a Neanderthal nuclear genome using bone pieces recovered from a Croatian cave. Comparisons with genomes from present-day populations provide direct evidence of interbreeding.
- Living people with European or East Asian ancestry carry approximately 1%–2% Neanderthal DNA.
- Native Sub-Saharan African populations lack Neanderthal DNA, indicating that their ancestors did not meet Neanderthals.
- Additional fossils and genome comparisons show that interbreeding occurred more than once.
What Neanderthals Contributed
- Two important immune-response genes appear to have entered Eurasian populations from Neanderthals.
- STAT2 helps the immune system signal a response to viral infection. Its Eurasian form is traced to Neanderthals because it is absent in Sub-Saharan Africans and appeared in Eurasian genomes long after the Homo sapiens–Neanderthal evolutionary split.
- HYAL2 supports skin-cell repair following exposure to UVB radiation. Common in many East Asian populations, its Neanderthal origin may have helped modern humans moving through Asia.
- A Neanderthal-derived variant promoting fast blood coagulation was useful before medical care, but it is now associated with greater blood-clot risk.
How do the changing advantages and disadvantages of inherited genes demonstrate that adaptation depends on environmental conditions?
Natural selection favors traits under particular conditions. A variant that once improved survival, such as rapid clotting without medical care, can become harmful when circumstances change.
Denisovans: A Lineage Revealed by DNA
A handful of fossils and their genomes uncovered the Denisovans, their repeated encounters with modern humans, and their contribution to high-altitude adaptation.
Denisovans joined the known hominin family tree only recently. Their 2008 discovery was nearly accidental, and researchers announced the genetic identification of the previously unknown group in 2010.
Discovery and Sparse Fossil Evidence
Scientists extracted ancient mitochondrial DNA from a pinky bone at Denisova Cave in southern Siberia. Since the cave had already produced evidence of Neanderthals and modern humans, they expected Neanderthal DNA. Instead, the sequence belonged to an unknown hominin.
- Six bones bearing the distinctive genetic signature are known from Denisova Cave: a pinky tip, three molars, a long-bone fragment, and part of a skullcap. They range from about 52,000 to 195,000 years old.
- A separate partial jaw from the Tibetan Plateau dates to at least 160,000 years ago and has also been identified as Denisovan.
- The group has no formal scientific name because there is no type specimen sufficiently complete for comparing later discoveries.
- Their extremely limited fossil record makes Denisovans a “ghost lineage.” Other fossils may remain unidentified, especially because most fossils yield no extractable DNA.
What Their Genomes Reveal
- Mitochondrial DNA, inherited through mothers, indicates that Denisovans and modern humans shared a last common ancestor about 1 million years ago.
- Most Denisovan nuclear DNA is closer to ours than their mitochondrial DNA is. One possible explanation is additional interbreeding with a hominin in our lineage, such as Homo erectus.
Modern populations in East and South Asia contain small Denisovan contributions, while some Melanesian populations in the southwest Pacific carry as much as 6%. Differences between living human DNA and ancient Denisovan genomes point to encounters with at least three Denisovan groups, so interbreeding occurred repeatedly and in multiple places.
The EPAS1 Advantage
Many people native to the Tibetan Plateau carry the gene EPAS1. It is associated with variation in hemoglobin concentration, which matters because efficient oxygen transport is vital at high elevations. Denisovans appear to have supplied this gene, and natural selection strongly favored it because it helped modern humans survive at altitude.
Suppose you are examining a newly found hominin bone. How would you investigate whether it came from a known group or an unfamiliar one such as the Denisovans?
Compare the fossil’s anatomy with established specimens, establish its age and location, and attempt to recover mitochondrial and nuclear DNA. Then compare any genetic sequence with genomes from known hominins and modern populations. A new lineage may remain informally named if no adequate type specimen exists.
Why Homo sapiens Remained
Climate, competition, population vulnerability, and interbreeding all help explain why Homo sapiens survived while other hominin populations disappeared.
Anthropologists continue to ask why Homo sapiens became the sole surviving hominin. No single explanation presented here settles the question; several factors may have operated together.
Possible Causes of Neanderthal Disappearance
- Climate change: Europe experienced episodes of severe cold and aridity between 44,000 and 40,000 years ago. These conditions may have reduced Neanderthal numbers, while habitat models suggest that climate shifts increasingly divided their environments.
- Pressure from modern humans: There is no direct evidence of violent or aggressive competition. Still, continued movement from Africa into Eurasia may have allowed more numerous Homo sapiens populations to displace or absorb Neanderthals.
- Technological and social advantages: Some explanations propose that superior clothing, more advanced technology, or long-distance trading relationships benefited modern humans.
- Neanderthal vulnerabilities: Genetic evidence points to smaller populations and more frequent inbreeding. The resulting loss of genetic diversity could have limited their ability to respond to environmental change.
The Uncertain Fate of Denisovans
Because Denisovans were identified so recently and left so few recognized fossils, their disappearance is less well understood. Climate shifts and encounters with Homo sapiens may have affected them in ways similar to the pressures faced by Neanderthals.
Adaptation Through Interbreeding
Interbreeding may itself explain part of our species’ success. Neanderthals and Denisovans had already adapted to environments that Homo sapiens entered, and some of their useful genetic variants passed into modern-human populations.
- Inherited variants helped Homo sapiens adjust to unfamiliar local environments.
- Denisovan ancestry aided life at high elevation, while Neanderthal ancestry supported responses to intense sunlight.
- Neanderthal-derived variants also contributed to viral-infection signaling and faster clotting after injury.
- Although those hominin populations disappeared as distinct groups, portions of their genomes remain in people today.
Why does adaptive introgression complicate an image of human evolution as one simple, straight line?
It shows that human lineages did not only replace one another in sequence. They met, interbred, and exchanged genes, so later populations inherited adaptations from several branches.
Review Your Understanding
Identify two anatomical traits that distinguish Neanderthals from Homo sapiens.
Possible answers include their shorter, stockier bodies with robust limbs; heavy brow ridges; large noses; or more oval braincases compared with the rounder Homo sapiens form.
About how much Neanderthal DNA occurs in living people of European and East Asian ancestry?
Approximately 1%–2%.
Describe two Neanderthal genetic contributions and state the benefit associated with each.
STAT2 contributes to immune signaling during viral infection, and HYAL2 assists skin-cell repair after UVB exposure. Rapid blood coagulation is another example that once helped after injury, although it now carries increased clot risk.
How did researchers first recognize Denisovans, and what makes their fossil history difficult to study?
They recognized a previously unknown hominin through mitochondrial DNA extracted from a Denisova Cave pinky bone. Very few genetically identified fossils exist, most fossils preserve no usable DNA, and there is no adequate type specimen.
Which Denisovan-derived gene occurs in many Tibetan Plateau populations, and how does it help?
EPAS1 is linked with hemoglobin concentration and supports efficient oxygen transport in high-altitude conditions.
Summarize the principal explanations proposed for the disappearance of Neanderthals.
The proposals include cold and dry climate cycles, fragmented habitats, demographic pressure from more numerous modern humans, possible technological or social advantages among Homo sapiens, and small Neanderthal populations with inbreeding and reduced genetic diversity.
Explain how interbreeding with other hominins supported Homo sapiens adaptation and survival.
Interbreeding transferred variants that other hominins had acquired in local environments. These genes helped modern humans cope with high elevations and UVB exposure and also affected immune signaling and blood clotting.
Common misconceptions
Watch out: Neanderthals were simple, unintelligent “cavemen.”
Instead: Evidence of fire control, tool and spear production, jewelry, and care for injured companions demonstrates sophisticated behavior.
Watch out: Homo sapiens survived because the species was naturally superior from the beginning.
Instead: The evidence presented allows several interacting explanations, including population size, climate, technology, social factors, and useful genes acquired from other hominins.
Watch out: Direct violence between modern humans and Neanderthals is established as the cause of Neanderthal extinction.
Instead: There is no direct evidence here for violent or aggressive competition. Researchers instead consider several demographic, environmental, technological, social, and genetic factors.
Lesson recap
- Neanderthals occupied a broad Eurasian range, displayed complex behavior, and interbred with Homo sapiens multiple times.
- Denisovans are known mainly through genetics and a small fossil sample, yet their DNA survives in several modern populations.
- Adaptive introgression preserved useful inherited variants, including genes connected with immunity, skin repair, clotting, and high-altitude oxygen transport.
- Climate stress, demographic competition, technology, social organization, small populations, and reduced diversity are among the explanations for why other hominins disappeared.
- The genetic legacy of Neanderthals and Denisovans remains within Homo sapiens and may have contributed substantially to our adaptation.