Long before Homo erectus trekked across Eurasia, eight different ape-human hybrids stood up and walked on two feet.
These eight distant ancestors lived between two million and 11.6 million years ago in a wide range of geographical locations from South Africa to Chad and southern Germany. All eight could be described as hybrid ape-humans, usually with small brains and the ability to swing from trees mixed with the ability to walk upright. Signs of bipedalism include changes to the pelvis, femurs, ankles and feet.
Our eight distant ancestors all feature a different mix of traits, but one thing ties them together: a network of overlapping global bird migration flyways. Fossil remains from all eight species were found where two or more bird migration flyways converge, typically on seasonal grounds. The convergence of two or more flyways is relatively unusual, meaning the correlation between ape-human hybrids and migratory birds is likely meaningful.
But there’s more to this remarkable story, going back much further into the past.
The busiest and biggest avian seasonal ground on the planet coincides with the region of central and west Africa where chimpanzees have lived for up to 40 million years. Separated from chimpanzee territory by the Congo River, bonobos evolved between 1.5 and 2 million years ago. Leading out of chimpanzee and bonobo territory are bird migration routes leading directly to the sites where bipedal primates have been discovered. Ornithologists believe bird migration routes don’t change over time.
The curious primate
Every year chimps and bonobos watched hundreds of millions of birds fly away beyond the horizon to some unknown destination. Six months later they all returned, filling the world again with color and birdsong. Did some group of intrepid primates give in to curiosity and follow them to find out where they went?
Although curiosity in animals hasn’t been widely studied it appears to coincide with highly adaptive “generalist” animals, including humans. In effect, curiosity is the desire to learn more about something intriguing in the environment. The question of where all those millions of colorful and musical birds went for six months a year certainly qualifies.
For more than a century scholars believed increased brain size drove the evolution of humans to become bipedal animals, but a flurry of fossil discoveries over the last 20 years has demonstrated that walking on two feet occurred millions of years earlier, and among species with small brains. Now it seems possible the opposite is true – that bipedalism somehow resulted in bigger brains.
Imitation is another dominant human trait, one that inspired Aristotle to write that we are “the most imitative creatures in the world … ” Agreeing with Aristotle is Andrew Meltzoff, co-director of the University of Washington Institute for Learning and Brain Sciences, who believes, “Human beings are the premier imitators on the planet.” Perhaps five different primate species imitated the fact that birds walk on two feet. Larger waterfowl have to run at full speed to gain lift.
A case of natural selection
There are several reasons why following migratory bird routes would have substantially improved the chances of surviving an extended trek.
Most importantly bird migration routes don’t stray far from fresh water and fertile ecosystems with plentiful resources. Avian seasonal grounds often coincide with volcanic areas that are perpetually recharged with minerals and feature drought-resistant ash layers.
Another important advantage of being among a high population of birds are their alarm calls, which warn of predators. Because birds can hear distant thunder and perceive changes in barometric pressure they engage in a sudden scattering behavior long before a catastrophic storm arrives. Finally, birds and their eggs make for nutritious meals. In the company of birds the primate trekker was more likely to find fresh water and food while enjoying a 24/7 security system that warned of predators and possible floods. It appears to be a classic case of natural selection. Those who lived and traveled among high avian populations were more likely to survive.
Chimps and bonobos
Chimpanzee and bonobo territory in equatorial Africa is the only place on the planet where seven bird migration routes from three global flyways overlap to produce a sky-high population and wide diversity of birds. The second busiest and biggest avian seasonal ground in the world is the adjacent Niger River Basin.
Migrating birds make it their business to follow the sun north and south because the benefits of living year-round in the warmest environment possible outweighs the heavy annual death toll caused by the hazards of migration.
But equatorial Africa is the ultimate avian destination – it’s sunny about 12 hours a day all year long. Maybe that’s why an unmatched number of migration routes across three (out of eight) global flyways dead-end there. Each day the sun shines down at about the same angle. The seasons don’t change much, although there are regular wet and dry periods.
The bird migration maps tell the story. Routes leading out of chimpanzee and bonobo territory lead directly to archaeological sites like the avian seasonal grounds at the Cradle of Humankind in South Africa and in northern Chad where Sahelanthropus tchadensis was bipedal more than six million years ago.
An intriguing wrinkle to the story is the fact that DNA sequences in humans diverged from bonobos and chimpanzees five to seven million years ago, the same time period when bipedal ape-human hybrids appeared in different parts of Africa. Both chimps and bonobos share 98.7 percent of their DNA with modern humans, making them our closest living relatives. The following descriptions of the eight hybrid species helps paint the picture of apes becoming human.
By far the oldest example of an ape-human hybrid was discovered in a clay pit in southern Germany in 2015, with a definitive study published in the November 2019 edition of Nature. A partial skeleton and an assortment of bones from at least three others were found.
Living 11.6 million years ago on the seasonal grounds of migratory birds in Hammershmeide, Germany, the bipedal Danuvius guggenmosi was “capable of both chimp-like suspension (from tree branches) and unassisted bipedalism,” according to paleoanthropologist David Begun of the University of Toronto.
The 2019 study found Danuvius guggenmosi was about the size of a baboon with long arms like a bonobo, and may be a common ancestor of both humans and great apes. The ability to travel through trees and on the ground would have allowed this particular primate to cover great distances through variable territory. While great apes favor their forelimbs and humans favor their legs, Danuvius guggenmosi struck a new balance, relying on both equally. The fossil evidence shows they made it all the way to Germany, presumably from Africa.
Living between 6 and 7 million years ago in what’s now the Djurab Desert in northern Chad, the bipedal Sahelanthropus tchadensis featured a combination of ape-like and human-like features. Its ape-like features were a sloping face, prominent brow ridges, an elongated skull and a small brain. Its human features included small teeth and a spinal cord opening located further forward than apes, allowing it to walk upright. The 2001 discovery of nine skulls showed for the first time that the earliest human-like species were more widely distributed than previously thought.
Living 5.7 to 6.1 million years ago in the Tugen Hills of Kenya, the bipedal Orrorin tugenensis shared traits with australopithecines (see below) but were more similar to modern humans. Three fragments from a femur bone led scientists to conclude the species was bipedal in a 2002 study published in Science Direct.
Living in the Middle Awash region of Ethiopia about 4.4 million years ago, Ardipithecus ramidus had feet similar to chimpanzees and gorillas and used all four limbs to travel over ground, but a 2019 study at New York University concluded that its elongated mid-foot and smaller hands reflected “an early form of bipedalism.”
Living 3.8 to 4.2 million years ago at Lake Turkana in Kenya and near Hadar, Ethiopia, Australopithecus anamensis walked upright thanks to a human-like ankle joint and also swung from trees thanks to their long forearms and flexible wrists. They had long, narrow and powerful jaws with heavily enameled teeth that allowed them to eat tough foods such as nuts and roots. Scholars believe they were the ancestors of Australopithecus afarensis.
Represented by the 1974 discovery of “Lucy” in Hadar, Ethiopia, the bipedal Australopithecus afarensis lived between 2.95 and 3.85 million years ago. Its 3.7-million-year-old footprints were found in volcanic ash in Laetoli, Tanzania. Similar to chimps, they grew quickly after birth and reached adulthood earlier than modern humans. They had an apelike face and a small brain with long, strong arms and curved fingers to climb trees. But scientists have concluded they regularly walked upright and could be the ancestors of later australopithecines.
Discovered in the Cradle of Humankind at the Sterkfontein caves in South Africa, the bipedal Australopithecus africanus lived between 2 and 3.2 million years ago. Their human-like traits included a rounded skull, larger brains and smaller teeth than earlier primates. Their feet, legs and hips showed they walked on two feet while their shoulders and hands remained adapted for climbing.
Dental studies showed they had a diet similar to modern chimpanzees, including fruit, plants, nuts, seeds, roots, insects and eggs. Scholars believe Australopithecus africanus is a viable candidate as the ancestor of Homo erectus.
Living in the Cradle of Humankind between 1.2 and 1.8 million years ago, the bipedal Paranthropus robustus was similar to australopithecines but had a wide, dish-shaped face and a large sagittal crest that anchored chewing muscles to the top of the skull. They were able to eat tough nuts, seeds, roots, and tubers, and may have used bones as tools to remove nutritious termites from their mounds.
The argument begins
Scholars would argue that the fossil record represents only a tiny fraction of animals that once roamed the earth and are only preserved in certain environments. In archaeologist Peter Bellwood’s book, The 5 Million Year Odyssey: The Human Journey from Ape to Agriculture (Princeton University Press, 2022), he wrote that early human species probably lived all over Africa.
But Bellwood’s speculation is a logical fallacy – the absence of fossils due to environmental conditions doesn’t mean the area was once populated. To assume that early humans lived all over Africa is shortchanging their intelligence – the evidence suggests they followed the busiest bird migration routes to the most bountiful ecosystems available.
The facts are 1) migratory birds flock to volcanic regions around the world partly because the perpetually rich soils and drought-resistant ash layers help vegetation flourish, 2) early humans had good reasons to follow migratory birds and/or live among high bird populations and 3) volcanic regions are effective at preserving fossils via volcanic ash, basaltic flows and volcanic breccia.
Perhaps the next step in this new branch of research would be a mathematical probability analysis. What are the chances that the correlation between ape-human hybrids and migratory bird routes is purely coincidental? And what are the chances the correlation is profoundly meaningful?
A final note
It’s worth noting that a 2016 study published in the Royal Society of Biology concluded that migratory birds transport enough exotic seeds to their seasonal grounds to substantially diversify the flora over time. Across ancient cultures migratory birds were celebrated in creation myths for delivering seeds to the earth and shaping the landscape at the beginning of time. While we can’t extrapolate these beliefs into the more distant past, it was probably fairly obvious that the most bountiful ecosystems correlated with the highest populations and widest diversity of birds.
(Ben H. Gagnon is an award-winning journalist and author of Church of Birds: an eco-history of myth and religion, released April 2023 by John Hunt Publishing. Order here or at other booksellers. More information can be found at this website, which links to a pair of YouTube videos written by the author and produced by JHP.)