Redefining Homo: Does our family tree need more branches?

by Mary Caperton Morton
Tuesday, August 16, 2016

The hominin family tree is frequently revised, but researchers disagree about where many fossils should be placed on the tree. Credit: K. Cantner, AGI.

As the sole surviving species of the genus Homo, we Homo sapiens are one of the most taxonomically lonely species living on Earth today. But dig back a few thousand years or more and we find ourselves with plenty of company: Many now-extinct species shared the genus Homo, ranging from the robust Homo neanderthalensis, to the hobbit-like Homo floresiensis to the more primitive Homo habilis and Homo erectus. But do all these species, with their wide diversity of physical and cultural traits, actually belong in the same genus?

Traditionally, hominin fossils have been classified into either the genus Homo or Australopithecus, with Homo dating back to about 2.8 million years and the oldest Australopiths dating back to about 4 million years ago. But some anthropologists think we need more options. “Right now, we are stuck in a false dichotomy, where if it isn’t an Australopith, it must be Homo and if it isn’t Homo, it must be an Australopith,” says Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York City. “We obviously need more genera if our classification of hominins is to meaningfully reflect the diversity within our family.”

But reparsing the hominin family tree is easier said than done. “This problem is just as much philosophical as taxonomical,” Tattersall says. “We’re wrestling with nothing less than human exceptionalism” — the idea that humans are so distinct from other organisms that the rules of taxonomy don’t apply to us, a problem that has plagued paleoanthropology from its earliest discoveries. “Homo has become a wastebasket of names with very little meaning,” Tattersall says. “And yet, we’re so emotionally attached to those names that even people who think they should be changed are unable to agree on how to go about it.” Nonetheless, some are trying.

What’s in a Name?

In the mid-1700s, Swedish botanist Carl Linnaeus proposed a binomial naming system to classify organisms according to relatedness and shared characteristics. This organizational system evolved into the familiar ranks of kingdom, phylum, class, order, family, genus and species.

A species is loosely defined as a population of organisms that can successfully breed. But things get a little more contrived at the genus level, which is less rooted in biology and more in the scientific drive for organization. “Species have a reasonably objective biological reality that is grounded in the dynamic that exists among their members,” Tattersall wrote in the journal Inference in February 2016. “Genera, on the other hand, are purely historical constructs,” he wrote.

“A genus is like a make of car,” says Bernard Wood, a paleoanthropologist at George Washington University in Washington, D.C. All Toyotas are more closely related to each other than to any other make of car and they’re all derived from the original Toyota, which was made in the 1930s, he adds. A grouping made up of “all the four-wheel-drive cars made by Toyota would make a sensible genus,” Wood says. “But a grouping of four-wheel-drive cars made by different companies would not qualify, even if they look alike and drive alike, since they don’t share a common ancestor.”

Three-dimensional skull casts of early hominins (left to right): Australopithecus africanus, 2.5 million years old from Sterkfontein in South Africa; Homo rudolfensis, 1.9 million years old from Koobi Fora, Kenya; Homo erectus, 1 million years old from Java, Indonesia; Homo heidelbergensis, 350,000 years old from Thessalonika, Greece; and Homo sapiens, 4,800 years old from Fish Hoek, South Africa. Credit: Smithsonian Institution.

Genera are not as easy to define when it comes to living organisms, however, especially those as complicated and poorly represented in the fossil record as early hominins. Prior to the mid-1900s, paleoanthropologists used taxonomic names to refer to particular fossil specimens, and by the mid-20th century they had racked up more than a dozen different genera, most of which were actually the same species: Homo neanderthalensis. “The first paleontologists were experts in anatomy, not taxonomy,” Tattersall says. “They were tuned in to these minor [anatomical] differences between specimens and named them not only as separate species, but entirely different genera.”

In 1963, taxonomist and ornithologist Ernst Mayr tried to set everybody straight by dragging the field in the opposite direction. “Mayr was the original lumper,” says Jeff Schwartz, a physical anthropologist at the University of Pittsburgh. Mayr was known as the leading architect of the new evolutionary synthesis, which held that evolution was a gradual process where species gave rise to the next through natural selection in a neatly linear fashion.

Despite never having seen a hominin fossil, Mayr declared there was only one genus in the hominin family — Homo — and that there were only three species, each giving way to the next in an orderly fashion: Homo transvaalensis (roughly equivalent to what we call Australopiths today) gave rise to Homo erectus, which paved the way for Homo sapiens to populate the planet.

“Mayr grouped members simply on the basis of bipedalism: everything bipedal was automatically Homo. He never looked at fossils and never looked at the literature,” Schwartz says. “It’s mind-boggling that the field of paleoanthropology is still being influenced to this day by a nonpaleoanthropologist. We’ve effectively been mired in Mayr’s misunderstanding since 1963.”

How Do You Define Homo?

When Carl Linnaeaus proposed a binomial naming system, he gave each plant and animal at least one qualifying characteristic that set them apart, but for humans, he wrote "Know thyself." And to this day, "Homo sapiens" has not been properly defined, researchers say. In 1769, anatomist Johann Friedrich Blumenbach compiled a list including a chin, small jaws, small canines abutting small incisors, a large braincase balanced atop a vertical vertebral column, erect posture, bipedalism, a bowl-shaped pelvic girdle, buttocks and a long thumb. In 1963, Ernst Mayr lumped all hominins into the same genus: Homo. More recently, paleoanthropologists tend to use one or more of four criteria to include hominins as members of Homo, as shown at left. Credit: K. Cantner, AGI.

When Linnaeus was laying out his case for a binomial naming system, he bestowed Latin genus and species names on a number of plants and animals, including humans. For each plant and animal, he gave at least one qualifying characteristic that set them apart, but for humans, he simply wrote, “Nosce te ipsum,” or “Know thyself.”

“To this day, Homo sapiens have not been properly defined,” Tattersall says, despite being the type fossil for Homo. Attempts to do so usually result in a grab bag of features that distinguish us from other mammals. In 1795, anatomist Johann Friedrich Blumenbach compiled a list including a chin, small jaws, small canines abutting small incisors, a large braincase balanced atop a vertical vertebral column, erect posture, bipedalism, a bowl-shaped pelvic girdle, buttocks and a long thumb.

Without a formal definition for even Homo sapiens, determining which fossils should fit into the genus is tricky, Wood says. “We’re looking for fossil species that are not identical to modern humans but are not so different from modern humans that you would want to put them in a different adaptive group.” Adaptive groups are made up of related organisms that share similar adaptations and lifestyles — such as marine mammals. “Arguments arise when you talk about how far away from modern humans you can get in terms of morphology while still maintaining an argument of adaptive coherence,” he says.

Paleoanthropologists have traditionally used one or more of four criteria to include hominins as members of Homo: an adult brain size of more than 600 cubic centimeters; limb, hand and foot proportions similar to modern humans; the ability to communicate through language; and the ability to manufacture stone tools. “However, none of these criteria are very satisfactory,” Tattersall says, especially since they have each been stretched and distorted repeatedly over the years to accommodate newly discovered fossils.

Brain size was the original preferred defining feature for Homo, says paleoanthropologist Rick Potts, director of the Smithsonian’s Human Origins Program at the National Museum of Natural History in Washington, D.C. “Brain size was fairly easy to measure, even given only fragments of the skull, and it presented a nice, linear trend: As Homo got more sophisticated, brain size got bigger.” But while brain size is useful in defining the later members of Homo, drawing the line between Australopiths and the earliest members of Homo is difficult. “Early members of the genus Homo had fairly small brains that overlapped with the brain size of Australopithecus,” Potts says. “If you’re going to define the genus Homo at its branching point from Australopiths, brain size is not the best way to do it.”

One of the traditional ways to distinguish Homo from other genuses is by brain size. Homo has an adult brain size of more than 600 cubic centimeters. Credit: K. Cantner, AGI, after Shultz and Mason, 2013.

Proportions of the limbs, hands and feet offer more clear-cut measurements to distinguish early Homo from Australopiths, which were smaller in stature and had limbs, hands and feet that were still geared toward an arboreal lifestyle in the trees. “For Australopiths, becoming bipedal was a real defining feature,” says Fred Spoor, a paleoanthropologist at the University College London in England. But limb proportions are quite similar between species of Homo, making it difficult to differentiate species based on their proportions. “Once we start seeing the modern body plan in early Homo, it’s difficult to draw the line between one species and another based simply on the limb proportions.”

Spoken language is another criterion that could be used to distinguish Homo, but while endocranial casts can offer some clues for the development of the language centers of the brain, definitive physical evidence for the origin of complex spoken language is thin. “All we can do is to look at the archaeological record and make inferences about whether speech would have been necessary for their level of culture and society,” Wood says. “That’s a long chain of inference so I’m reluctant to use complex spoken language as a criterion for Homo.”

The ability to manufacture stone tools is a common way to differentiate Homo from other hominins. These handaxes are from early hominins. Left to right: Africa (1.6 million years old), Asia (1.1 million years old), and Europe (250,000 years old). Credit: Chip Clark, Smithsonian Institution.

In the 1960s, the idea of defining Homo according to tool use was very much in vogue. “‘Man the toolmaker' was an interesting notion at the time, but like a lot of interesting notions, it’s a hypothesis that has probably been refuted,” Wood says. “The tool use criterion is not very helpful because it’s quite likely that Australopithecus also used tools. Even chimps use tools, though they are less adept at actually making tools, but it seems like the cognitive basis for tool use was present in the common ancestor of chimps and modern humans.”

Over the years, people have proposed alternative criteria for defining Homo, some more helpful than others. Tooth size is fairly useful, especially since teeth are some of the most commonly preserved parts of the skeleton. “Early hominins had much larger teeth than we do relative to our body size,” Wood says. Paranthropus, for example, is a genus of early hominins distinguished by their significantly larger teeth — so large the first specimen of Paranthropus boisei, discovered in 1959, was nicknamed “Nutcracker Man.” As early hominins developed stone tools, they were able to process foods outside the mouth and the selection for large teeth diminished.

Others have proposed using developmental growth as a criterion, as humans and apes have very different growth patterns. “Modern humans have a childhood and a prolonged growth period, but if you look at chimps, they go from being infants to young adults in a number of weeks,” Wood says.

Perhaps the most radical idea for grouping species into genera is on the basis of time, with lines drawn between species based on when they evolved and how long they survived. “It’s a perfectly logical solution to the problem,” says Mark Collard, an anthropologist at Simon Fraser University in Burnaby, British Columbia. “But one of the consequences is that we end up with modern chimps and gorillas being classified as Homo. I don’t think many people in the field feel comfortable with that, though I’m not sure they’re willing or able to articulate why.”

Not all paleoanthropologists agree that the genus Homo needs to be changed or redefined. Some see the creation of more genera as merely overcomplicating an arbitrary naming system. “When we talk about how species should be in a genus or a family, it’s kind of like asking, ‘How high is up?'” says Brian Villmoare, a paleoanthropologist at the University of Nevada, Las Vegas. “Nature does its thing. It finds new niches and lineages split. We’ve invented this system of taxonomy for organizational purposes and it’s essentially arbitrary,” Villmoare says. Creating new genera doesn’t add new information; rather, it just adds demarcations “somewhere on a continuum,” he says. “I don’t see the value in that.”

Rebranding Homo

Comparison of skull features of replicas of Homo habilis (1.8 million years old, from Koobi Fora, Kenya), Homo erectus (1.8 million years old, from Dmanisi, Georgia), Homo floresiensis (from Liang Bua, Flores, Indonesia, perhaps 20,000 years old) and Homo naledi (age unknown, from South Africa). Actual fragments of Homo naledi material were overlaid on a virtual reconstruction. Credit: Chris Stringer, eLIFE, September 2015, CC BY 4.0.

If scientists were going to rebrand Homo, who would be “in” and who would be “out”? “I would allow Homo erectus in, but I would draw the line at Homo habilis,” Wood says. Homo habilis is the original problem child of the paleoanthropological community. Described in 1964 by Louis Leakey and colleagues, the team named their find the “handy man” in reference to a collection of stone tools found in association with the fossils, which dated to about 1.75 million years ago. At the time, the “man the toolmaker” hypothesis was in vogue, with many paleoanthropologists enthralled with the idea that tool use should be the defining feature of the genus Homo.

Homo sapiens skull features, for comparison. Credit: K. Cantner, AGI; skull: © Pangcharoen.

But Homo habilis is more similar in size and body type to Australopithecus. The original fossil described by Leakey had a brain slightly bigger than the average Australopith, but the brains of subsequent specimens have been smaller, between 340 and 500 cubic centimeters. “If you include Homo habilis in Homo, you end up including a species that had limb proportions that are closer to Australopithecus, as well as dentition and a jaw whose scaling in relation to body mass is closer to Australopithecus and a brain size closer to Australopithecus,” Wood says. “A good place to draw the line” is between Australopiths and Homo, he says, adding that Homo rudolfensis, a species known from a handful of fossils found in Kenya, should also be left out of genus Homo.

However, just because Homo habilis does not belong in Homo doesn’t mean it’s an Australopith, Tattersall says. “My sense is that Homo habilis should belong to its own genus, neither Australopith nor human,” he says. “Chucking out Homo habilis makes sense because it makes Homo a morphological mess. It certainly tidies up the [Homo] genus, but stuffing it into Australopithecus just reinforces this notion that if something is not an Australopith it has to be Homo and vice versa. We need a bit more taxonomic elbow room to express the simple variety of morphology we see in the hominin fossil record.”

But this problem won’t necessarily be solved simply by coining a new name. “We need a whole new approach,” Schwartz says. “I say we start from scratch. Forget the names. Once you give something a name, you and the specimen are stuck with it. Instead of deciding how many new genera we need from the beginning, let’s start with the fossils, like you would with any other species,” Schwartz says.

A large part of the problem is that paleoanthropologists are unwilling to rebrand fossils that have already been named, Tattersall says. “Anybody who has tried to rename an existing fossil that has already been described has had a very hard time being taken seriously,” he says. The only person who has proposed a new genus lately for a fossil between Australopiths and Homo is Meave Leakey, who coined the genus Kenyanthropus for a new species she discovered in northern Kenya in 1999. “She knew it wasn’t Homo and it wasn’t an Australopith either so she came to the logical conclusion that it was something else. She got away with it because she had a new fossil, but she got dumped on considerably for it.”

Younger fossils are also trapped in a dichotomy, where if it isn’t a Neanderthal, it must be a Homo sapiens, Schwartz says. “Until about 20,000 years ago, there were Neanderthals and Homo sapiens and after then there were only Homo sapiens. But we’re finding a lot of variants in Asia, particularly China and Indonesia, and we keep classifying everything as Homo, regardless of what they look like,” he says. “A different picture is emerging of a lot more diversity in Asia in recent times than we see in Europe and Africa. And we need to stop lumping everything into Homo sapiens if we’re going to explore that diversity properly.”

The problem is highlighted by the so-called hobbits found on the Indonesian island of Flores in 2003. Standing just over a meter tall with small brains and large teeth, the hobbits defy the classical definition of Homo, and yet they have been classified as Homo floresiensis. “It’s really problematic, assigning those hobbits to Homo, when we’re talking about something that’s a meter tall, with a tiny brain by hominin standards,” Collard says. “It’s very difficult to see how that doesn’t stretch the boundaries of the genus Homo beyond what should be reasonable.”

Beating Around the Bush

Paleoanthropologists argue about whether Homo floresiensis, which stood just about a meter tall and had a much smaller brain than modern Homo sapiens, but lived very recently, should be classified as Homo. Homo floresiensis remains have been found in the Liang Bua Cave on the Indonesian island of Flores. Credit: Liang Bua Team.

In the 18th century, when Linnaeus christened mankind as the genus Homo, people didn’t think humans had a fossil record. The history of our species was rooted in the creation story, not evolution. But nearly 300 years later, paleoanthropology is still tiptoeing around the imaginary boundary between humans and animals. “This issue plays into several big-picture, profound and philosophical questions that paleoanthropologists are often unconsciously butted up against,” Collard says. “It’s something we’ve struggled with for hundreds of years. Not just in paleoanthropology but also Western science and philosophy.”

The problem is thrown into a harsher light when comparing the classification of Homo with any other animal group, Spoor says. “For a paleontologist who studies any other animal group, there would be no hesitation to give new genus names to groups of species to comply with the rules of taxonomy. Within human evolution, these rules are constantly being violated.”

For example, before molecular testing, paleontologists could only speculate where whales and dolphins should fit into the mammalian family tree. “Then genetics came along and showed they are most closely related to hippos and even-toed ungulates, and the entire order was reclassified to reflect that radical shift with very little controversy or resistance,” Spoor says. “However, in human evolution, we’re so sensitive to creating scandal and even minor confusion that there’s great reluctance, even in papers that beautifully address this problem, to offer solutions.”

In the future, genetics may offer deeper insights into the relatedness of hominin species, but currently, the technology is limited, especially when analyzing older fossils. “A decade ago, we didn’t think there could be a Neanderthal genome project and now there is,” Potts says. “Who knows where that technology will take our genus in the future.”

Until then, paleoanthropologists will keep working on finding new fossils and placing them within the branches of the human family tree, which gets more bush-like with every new discovery. “Sometimes new fossils bring more clarity and sometimes they bring more confusion,” Wood says. “We will need to keep getting smarter about how we interpret these fossils, how we view our ancestors and how we view ourselves.”

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