Defining Homo sapiens
March 11, 2026
Homo sapiens are, of course, humans. But there's no easy way to distinguish ourselves from other similar species.
For animals, the most intuitive way to define a species is the "biological species" concept. It says that a species is the largest group of organisms capable of reproducing with one another and producing fertile offspring. While dogs and cats can't reproduce, horses and donkeys can, but they produce sterile offspring in almost all cases[1]. For other animals, the line between species gets blurrier. Lions and tigers can also reproduce, and while their male offspring are almost always sterile, their female offspring are typically fertile[2]. For Homo sapiens though, the biological species concept breaks down completely. Every living human has some Neanderthal DNA (Homo neanderthalensis), so we clearly interbred with them and produced fertile offspring.
The next most logical way to distinguish Homo sapiens from our close relatives would be to classify each group by its ecological niche. But, beyond the fact that we are land-dwelling animals, Homo sapiens are clearly not restricted to a single ecological niche. And for our close relatives like Neanderthals, evidence continues to point in the same direction, including remains stretching from Portugal to Italy that suggest many groups developed specialized marine diets of fish, shellfish, and marine birds and mammals. This contradicts the stereotype of Neanderthals as a cold-weather species restricted to hunting large game on the Western Eurasian steppe.
Many existing human populations also have DNA from other members of the genus Homo, including Denisovans and so-called "ghost populations", whose DNA appear in some human genomes but has not been conclusively matched to a specific member of the genus. The genomes of some West African and South Asian populations, for example, reveal the existence of ghost populations. Researchers have suggested Homo ergaster, Homo floresiensis, Homo luzonensis, or otherwise undiscovered archaic hominins as the potential identity of one or more of these ghost populations.
Many papers, articles, and news reports refer to members of the genus Homo as distinct "human species". Referring to them as species doesn't align with the biological species concept. Plus, if they were indeed human, then Homo sapiens are only a subset of all humans. One reason for the disconnect is that paleoanthropology is an older field of science than genetics. Long before we understood DNA and could prove that modern humans interbred with other hominins, paleoanthropologists were classifying them primarily according to their anatomy. It's also extremely difficult to extract undamaged bits of DNA from many hominin fossils, due to the exceptional antiquity of the remains or the environment they were preserved in. As a result, anatomy is still the most reliable tool for categorizing hominins.
Anatomy is inferred primarily from bones and teeth, since these organs are most likely to fossilize. This leaves large gaps in our understanding of archaic hominins' soft tissue anatomy, but it does lead to solid conclusions about height and muscle mass, as well as brain size and brain shape. Jaw and tooth structure can also tell us something about a hominin's diet, but, given the incredible variation in modern and ancient Homo sapiens diets, it often doesn't strongly suggest any particularly unique characteristics. Interestingly, Homo sapiens are the only apes with a "true" chin. All other apes, including more distant relatives such as chimpanzees and gorillas, lack the same bony structure on the front of their mandibles. This is known as an autapomorphic trait, a trait that is unique to one group of organisms and absent from all others. Autoapomorphic traits are a key way to distinguish species by their anatomy.
Nevertheless, the study of anatomy actually highlights just how similar Homo sapiens are to other hominins. Of all our ancient relatives, Neanderthals are the most well-known and well-studied. The genomes of modern humans are 99.7% identical to those of Neanderthals, leaving only 0.3% where alleles, genes, indels, and non-coding sequences of DNA commonly differ. Notably, this is the same level of genetic differentiation observed between isolated populations of chimpanzees, yet they are considered the same species. For additional context on genetic differentiation, we share 99.9% of our genomes with all living humans, 98.8% with chimpanzees, about 85% with mice, and about 70-75% with elephants. Neanderthals are commonly characterized as slightly shorter than Homo sapiens and as having more muscular bodies, a large nasal aperture, a prominent brow ridge, and a 10% larger brain with a much bigger occipital lobe and a smaller frontal lobe. But many Neanderthal traits lie within the range of normal Homo sapiens variability. For example, average height of an adult male ranges from under five feet in populations of present-day Congo to above six feet in various regions in Europe. Neanderthals, on the other hand, were thought to be about five and a half feet tall, a few inches shorter than the average Homo sapiens they coexisted with. Studies have suggested that, despite their shorter stature, Neanderthals had somewhere between 5-15% more[3] lean muscle mass[4] than Homo sapiens. This is similar to the level of variation in muscle mass observed across broad ethnic groups in the US[5], especially in aging individuals. Additionally, the modern human populations with the highest muscle mass have Central and West African ancestry, ethnicities which have less Neanderthal DNA than almost all other Homo sapiens populations. Arctic-dwelling modern humans, such as the Inuit and Yupik peoples, also have certain cold-weather adaptations that show a degree of similarity to Neanderthals, such as shorter limbs relative to torso size, greater lung capacity, and a larger nasal aperture. Current evidence suggests these morphological traits are examples of convergent evolution, having developed independently rather than as the result of Neanderthal ancestry[6].
Consider also the Red Deer Cave people, whose fossils were discovered in Southwest China in 1979, dated to as recently as 11,500 years ago. Since their discovery, researchers have suggested their morphology resembles Denisovans or much older hominins such as Homo erectus or Homo habilis. Recent DNA analysis, although debated, suggests they are just Homo sapiens with an uncommon morphology.
The distinction between Homo sapiens and our hominin relatives is further complicated by the fact that a large degree of human variability is due to phenotypic plasticity, the ability of a single genotype to produce different observable traits (phenotypes) depending on environmental conditions. One well-known example of phenotypic plasticity comes from a study of London taxi drivers that found that the average taxi driver's posterior hippocampus, a region of the brain involved in spatial navigation, grew 7-10% larger than average, and its size correlated with the number of years an individual had worked as a taxi driver. It's possible that for the many Neanderthal groups that lived through long, dark winters and hunted large game, phenotypic plasticity can partially explain some of their traits, like greater muscle mass and a larger occipital lobe, the region of the brain responsible for visual processing[7]. In fact, a 2014 study of Neanderthal and Denisovan epigenetics suggested that gene expression was the primary driver of skeletal changes across these hominin populations, rather than underlying genetics that differed from Homo sapiens. That said, I'm definitely not suggesting that phenotypic plasticity is the sole explanation or a better explanation for any given trait that differs across archaic hominin populations. My point is that it's extremely difficult to know what portion of a physical trait is due to unique genetics and what portion is due to culturally-ingrained behaviors and/or the environmentally-regulated expression of shared genetics.
The differences observed between archaic hominins are further complicated by small sample sizes. Researchers estimate that the fossil record currently contains about 400 individual Neanderthals, only 30 of which are relatively complete specimens. Neanderthals are thought to have had extremely low population density, perhaps never exceeding 70,000 individuals, so 400 individuals may be a representative sample, though it's certainly debatable. And, given that they roamed the earth for at least 375,000 years and were spread across Europe, the Middle East, and Asia, it would not be surprising if Neanderthal genetics, appearances, and culture were at least as varied as they are across modern human ethnic groups.
With regard to other archaic hominins, like Denisovans, it's clear that the limited number of findings are not representative samples. In fact, we only discovered that Denisovans existed in 2010, due to DNA analysis of a bone fragment. Because so little is known about them, they don't even have an official species name; Homo longi and Homo denisovensis have been suggested. The remains of about 15 individual Denisovans have been confirmed to date, and about 10 of them come from Denisova Cave in Siberia, each of which is linked to a single tooth, a small bone, or a bone fragment. Outside of Denisova Cave, a single skull, two mandibles, a tooth, and a rib have been confirmed as Denisovan remains, all discovered in East Asia. If, for example, a number of these Denisovans experienced famine in utero, their physical characteristics would have been highly skewed relative to the average Denisovan. And the modern human populations with the highest amount of Denisovan DNA are indigenous to the Philippines, Papua New Guinea, and Australia. It's possible that modern human populations migrated there long after mingling with Denisovans, but if Denisovans did inhabit warm equatorial regions that far south, their physical characteristics might have differed considerably from Siberian Denisovans.
Despite our anatomical similarities, and the way phenotypic plasticity and small sample sizes blur the lines between groups, I am not suggesting that Homo sapiens are the same as our close hominin relatives. No modern human populations, with the possible exception of the Red Deer Cave people, share all or most of the physical traits of any known archaic hominin. Yet we are much more similar than commonly thought, and our categorization as different species is a weak distinction. Homo sapiens appeared at least 300,000 years ago, but our cultural complexity, marked by higher population density, superior weapons, expanded trade networks, and better cave art, is thought to have become widespread only 45,000 years ago. This doesn't suggest our relatives were inferior or incapable of producing similar cultural aspects; evidence suggests that, at least in isolated cases, many did create artifacts of equal symbolic or technological quality[8]. The widespread cultural complexity of Homo sapiens appears in the archeological record at the same time that all of our known remaining relatives began to die out (see Figure 1). If competition with Homo sapiens was a primary driver, then the disappearance of our relatives, although spanning thousands of years, resembles a similar pattern within Homo sapiens populations, where societies with superior weapons, trade, and art often intentionally or inadvertently wipe out others. But the cultural complexity of one society of modern humans does not imply that another is incapable of adopting such complexity or developing it independently.
In 1859, Charles Darwin wrote in The Origin of Species, "I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other". This is how we define ourselves as Homo sapiens. An alternative view of our past is that Neanderthals, Denisovans, and other archaic hominins were not different species that went extinct. They were more like highly isolated races of humanity that died out, and, given that all living humans carry some DNA from these archaic hominins, it's questionable whether they truly did die out. Ultimately, Homo sapiens are indeed humans, but, specifically, we're just the humans that survived.
Notes:
The Romans even had a saying, "cum mula peperit" (English: "when a mule foals"), signifying an impossible event, similar to English expressions like "When hell freezes over" or "When pigs fly". Interestingly, there are rare documented cases where female mules have produced offspring, but these are akin to a natural form of two wrongs making a right, where two subsequent errors cancel each other out. They result from an anomaly in the reproducing individual's own genetic makeup, or from a form of meiotic failure which skips genetic recombination and creates an egg with DNA from only one parent (100% horse or 100% donkey), which is therefore fertile.
This is consistent with Haldane's rule.
See Results, page 105, "discrepancy between Neandertal and anatomically modern human average body size". Also see Discussion, page 108, "Greater body mass in Neandertals compared to anatomically modern humans results mainly from reduced muscularity in the latter".
See Table 3.
See Figure 3.
This is consistent with Allen's rule. Also, there is evidence that Inuit people inherited adaptive cold-weather metabolic traits from Denisovans.
The same adaptation may exist to a lesser degree in modern humans. A 2011 study found that populations in higher latitudes tend to have larger eyes, suggesting they also have proportionally larger visual cortices.
Examples include a 40,000+ year old stone bracelet from Denisova Cave and the 400,000 year old Schöningen Spears.