Tiny fossils give clues to origins of human social structure
Did our ancestors lose grooming claws because we had each other to remove parasites?
June 22, 2018
When you’re caught without a comb on a windy day, you might wish for fingernails that could untangle your hair just as well.
A new study suggests that our distant ancestors actually had such nails, known as grooming claws. And the evidence comes from 45-year-old bags of sand Patricia Holroyd spent a decade combing through in UC Berkeley’s Museum of Paleontology. Holroyd is a senior museum scientist in charge of vertebrate collections.
Scientists used to think that monkeys, apes and humans developed only flat finger and toe nails, while other primates — lemurs, lorises, galagoes and tarsiers — developed nails plus a grooming claw on one or more toes for removing parasites. Most had a grooming claw on their second toe, while tarsiers evolved to have them on both the second and third toes.
But Doug Boyer, an associate professor in the department of evolutionary anthropology at Duke University, visited the Berkeley museum in search of primate fossils and discovered the nail and grooming claw toe bones of a 56-million-year-old primate ancestor, a mouse-sized animal called Teilhardina brandti. This is the oldest known primate from North America.
Digital scans of early primate toe tips, showing the difference between the bone underlying a grooming claw (left, Arapahovius gazini) and the bone underlying a flat nail (Omomyidae incertae sedis). (UCMP images from Morphosource)
Boyer teamed up with Jonathan Bloch, a Florida Museum of Natural History curator of vertebrate paleontology at the University of Florida, who had similar fossils. Together, Boyer, Bloch and Holroyd concluded that the common ancestor of all primates, not just the lemurs and lorises, had grooming claws. Monkeys, apes and humans eventually lost theirs, perhaps because we had each other to pick the lice and ticks from our hair.
“The loss of grooming claws is probably a reflection of more complex social networks and increased social grooming,” Boyer said. “You’re less reliant on yourself.”
“It has been commonly thought that loss of grooming claws may be associated with increased sociality in early primates, as they began to live in larger groups where members groomed one another, and to develop more precise grasping,” said Holroyd, a senior museum scientist.
Boyer, Bloch, Holroyd and their colleagues at UC Berkeley and the Florida Museum of Natural History published their findings this week in the Journal of Human Evolution.
Bags of sediment
According to Holroyd, more than half the fossils described in the paper were originally collected in bags of sediment in the 1970s by crews from UC Berkeley led by the late paleontology professor Don Savage, who was working in the Washakie Basin of southwest Wyoming on mammal evolution. Most of these bags sat in storage until Holroyd arrived, when she and her students began slowly picking through the sediment to find minuscule fossils among the grains of sand. The gleaning took about 10 years, and she is now curating the fossils with the support of an Institute of Museum and Library Services grant.
The foot of a greater slow loris, Nycticebus coucang, showing a grooming claw on the second toe and flat nails on all the other toes. (Florida Museum photo by Kristen Grace)
Boyer, an expert on primate skeletons, came in 2013 to sort through the bones Holroyd found to locate ones that were likely to be primates. Holroyd provided information on the age and geologic setting and which primate species were known from teeth at each site where the bones were found, and which other mammals, especially tiny insectivores, might be similar enough in size to be confused with primates.
They found tiny bones called distal phalanges, the bones at the tips of fingers and toes, from omomyoids, the ancestors of monkeys, apes, humans and tarsiers, though not the branch of primates that gave rise to lemurs, lorises and galagoes. The shapes of these bones reveal whether they support a claw or nail. Bones topped with a claw mimic its narrow, tapered structure, while bones undergirding a nail are flat and wide.
Boyer and Holroyd combined this information with a smaller data set from the Bighorn Basin collected by Bloch, and data from a younger site Bloch and Boyer had explored and which spanned nearly 10 million years of early primate history.
Together, the fossils suggest grooming claws were hallmark features of the earliest primates, dating back at least 56 million years.
Holroyd notes that Teilhardina brandti is not the only animal in Berkeley’s collections with a grooming claw. She and her colleagues were able to show that at least four different species of early tarsier relatives also possessed grooming claws, demonstrating that this trait was widespread among early primates.
“We have long known that fossils in the lemur part of the primate tree have such claws, but it was assumed that early representatives of the clade comprising tarsiers and anthropoids, including humans, had already evolved flat nails.,” she said. “The fact that multiple taxa in the early branches of our own lineage retain this primitive trait is a novel finding and suggests that the evolution of fully flattened nails and associated behaviors occurred more recently than previously thought.”
The findings prove the value of collecting and curating a broad range of fossil-rich material, even when sorting through it can take decades.
“When I was staring through a microscope in the early 2000’s, picking through matrix to find the tiny bones, I did not yet imagine that they we would be able to make these kinds of associations and help change our understanding of early primate evolution,” Holroyd said.
Anyone can freely download the original, 3D micro-CT scans of the fossils from the Morphosource website, http://morphosource.org.
“The future of morphological research is in the digital realm, and making data on museum specimens freely available is key to that future,” she said.
Other co-authors are Stephanie Maiolino from Stony Brook University and Paul Morse from the University of Florida. The National Science Foundation and the Institute of Museum and Library Services provided funding for the research.
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