At least five other human species walked the Earth alongside or before Homo sapiens, and none survived. The reasons aren’t simple. A combination of climate instability, small population sizes, disease, and direct competition with our own species drove them all to extinction at different times and in different places. In some cases, they didn’t disappear entirely. Parts of their DNA live on in us.
The Other Humans
The genus Homo produced several species that overlapped in time with modern humans. Neanderthals appeared between 200,000 and 250,000 years ago and lived across Europe and western Asia until roughly 40,000 years ago. Denisovans, closely related to Neanderthals, occupied large parts of Asia for possibly more than 110,000 years. Homo heidelbergensis, the common ancestor of Neanderthals, Denisovans, and Homo sapiens, lived in Eurasia as early as 350,000 to 600,000 years ago. Homo floresiensis, a small-brained species discovered on the Indonesian island of Flores, survived until about 50,000 years ago. And Homo erectus, one of the longest-lived human species, persisted in Southeast Asia long after vanishing from most of its former range.
All of these species were successful in their own right. Neanderthals were skilled hunters adapted to cold climates. Denisovans spread across a vast geographic area. Homo floresiensis survived on a remote island for hundreds of thousands of years. Their extinctions weren’t inevitable, and no single cause explains them all.
Climate Instability Shrank Their Worlds
The period when most of these species disappeared, roughly 60,000 to 30,000 years ago, was one of extreme climate instability. During what scientists call Marine Isotope Stage 3, the Northern Hemisphere experienced 19 rapid warming-and-cooling cycles. Warming phases lasted about 60 years, while cooling phases could drag on for more than 2,000 years. These weren’t gradual shifts. They were whiplash-fast changes that repeatedly reshaped habitats.
For Neanderthals, this meant their suitable habitat in Europe repeatedly shrank and fragmented during cold periods, then expanded during warmer intervals. Southwest Europe, particularly the Iberian Peninsula, served as a core refuge. But each contraction pushed populations into smaller pockets, and the final contraction left them cornered in Iberia with no recovery. A 2024 study in the journal One Earth found statistically robust evidence that Homo erectus, Homo heidelbergensis, and Neanderthals all lost a significant portion of their climate-suitable habitat just before going extinct.
On Flores, freshwater availability played a critical role. Homo floresiensis disappeared from the fossil record around 50,000 years ago, and recent research tracing water availability at their primary site, Liang Bua, suggests that shifts in monsoon patterns may have dried out the resources they depended on. Their disappearance actually preceded modern human arrival at the site by several thousand years, which means climate change alone may have been enough to finish them off.
Small Populations, Big Problems
Neanderthals had lower genetic diversity than any living human population today. Their estimated effective population size was around 1,000 individuals, compared to roughly 10,000 for contemporary Homo sapiens. This wasn’t a temporary dip. Neanderthals experienced a prolonged population bottleneck lasting about 10 times longer than the bottleneck modern humans went through when migrating out of Africa.
Small populations accumulate harmful mutations faster because natural selection is less efficient at weeding them out. Genetic modeling estimates that, as a result, the average Neanderthal had roughly 40% lower biological fitness than the average human. Under some models, that figure drops to 61% lower fitness. In practical terms, this would have meant more health problems, lower fertility, and reduced ability to bounce back from population crashes caused by harsh winters, food shortages, or disease outbreaks. Every climate downturn hit harder because there were fewer individuals to absorb the blow.
Denisovans faced similar constraints. Their genetic variation was low compared to Homo sapiens, even though they occupied a wide geographic range across Asia. Low diversity in a spread-out population suggests they lived in small, isolated groups that rarely mixed with one another.
Diseases That Crossed Species Lines
When Homo sapiens migrated out of Africa and encountered Neanderthals and Denisovans, they didn’t just compete for food and territory. They also had sex, and along with bodily fluids, they likely transferred diseases. Researchers at the University of Cambridge have identified several infections that probably jumped from modern humans to Neanderthals: tapeworm, tuberculosis, the bacterium that causes stomach ulcers, and genital herpes.
These aren’t the kind of diseases that kill quickly. They’re chronic infections that slowly weaken a person, making it harder to hunt, travel, and reproduce. For small populations of hunter-gatherers already under pressure from climate change and competition, even a modest reduction in overall health could tip the balance toward extinction. Neanderthals had no prior exposure to African-origin pathogens, so they would have lacked the immune defenses that Homo sapiens had built up over thousands of generations.
Competition With Homo Sapiens
Even massive natural disasters couldn’t do what Homo sapiens did. Around 40,000 years ago, the Campanian Ignimbrite eruption in southern Italy, the largest volcanic event in the Mediterranean in 200,000 years, blanketed central and eastern Europe in 250 to 300 cubic kilometers of ash. It happened during an already brutal cold period, and the combination likely triggered a volcanic winter. Yet archaeological evidence shows that neither Neanderthals nor early modern humans were wiped out by it. The eruption failed to have lasting impacts on either population. What did have lasting impact was the continued presence of Homo sapiens. The researchers who studied the eruption’s effects concluded that modern humans proved a greater competitive threat to indigenous populations than natural disasters.
How exactly did Homo sapiens outcompete other humans? Several advantages compounded. Sapiens populations were larger, which meant more genetic diversity, more innovation, and more resilience to setbacks. Studies across multiple fields suggest Neanderthals were actually more cooperative and less aggressive than Homo sapiens, which may have been a disadvantage when the two groups clashed over the same resources. Sapiens also had wider social networks and more varied diets, making them more adaptable when environments shifted.
For Homo floresiensis, the competition story is less clear. On Flores, their disappearance around 50,000 years ago may have preceded direct contact with modern humans at their main cave site. But elsewhere on the island, the timeline remains uncertain. The broader pattern still holds: once Homo sapiens developed the ability to cross water and reach remote islands, isolated species like floresiensis lost their last safe haven.
Not Fully Extinct: DNA That Survived
The extinction of other human species wasn’t always a clean break. Neanderthals and Denisovans interbred with Homo sapiens frequently enough that their DNA persists in living people today. Most non-African populations carry about 1 to 1.5% Neanderthal DNA across their genomes. Denisovan ancestry is highest in people of Oceanian descent, where it reaches roughly 0.85% on average, and some populations carry up to 5%.
This genetic legacy is uneven. The X chromosome carries far less archaic DNA than the rest of the genome, which suggests that hybrid offspring with archaic X-linked genes had lower fertility or survival. In other words, natural selection kept some Neanderthal and Denisovan DNA while actively purging the rest. The Neanderthal DNA that remains in modern humans still carries a small cost: non-African populations are estimated to have about a 0.5% fitness reduction from residual Neanderthal mutations.
So the other humans didn’t all vanish without a trace. Some were absorbed. Neanderthals and Denisovans live on as fragments of code scattered through billions of people. But as independent species, with their own cultures and ways of life, they’re gone. The combination of unstable climates, tiny populations, new diseases, and an aggressive, adaptable competitor proved too much for any of them to survive on their own terms.