gay nalediIts brain may have been small, but it was surprisingly similar to ours, according to a new study that suggests structure in hominin brain evolution may be earlier than size.
Measurements of skull fragments indicate that: gay nalediIts brain was about the same size as that of an Australopithecine – the genus of primates that lived in Africa 2 to 4 million years ago and may belong to our early ancestors. Yet the diminutive species was still present in Africa long after the Homo lineage appeared and may have overlapped with modern humans. So how it fits into our family tree is not clear.
A new study finds that, despite its size, gay naledi‘s brain looked very different than Australopithecus‘ and many more like ours, at least in some very important areas.
In some ways, the brain structure of an Australopithecine was very similar to that of modern great apes. Human brains differ mainly in structures related to language and social skills. The fact that gay nalediIt’s so much more like our hints that those changes happened early in the history of our lineage – closer to the base of our branch of the family tree.
Lover frontal lobes
How do we know what the brains of extinct hominins looked like? The inside of the skull sometimes retains an impression of the outer surface of the brain, so making a cast of the inner surface of the skull (paleoanthropologists call this an endocast) will give you a good idea of the animal’s brain shape. . Paleoanthropologists have studied endocasts of two Australopithecine species and a handful Homo species – and now they’ve expanded that list to include: gay naledi.
In casts of Australopithecus sediba (2 million years ago) and Australopithecus africanus (3.3 to 2.1 million years ago) skulls, we can see a structure in the front of the brain called the fronto-orbital sulcus. But in modern humans, the frontal lobes of the brain have expanded posteriorly and to the sides, creating structures called the frontal opercula.
The frontal opercula contain different structures with roles in language, social behavior and the kinds of motor planning you need to make (for example) stone tools. Those structures include Broca’s area, a structure on the left side of the brain that is strongly associated with the ability to produce speech.
Frontal opercula appear in endocasts of some early Neanderthal relatives from Sima de los Huesos in Spain and even in some homo erectus (1.5 million years ago) endocasts. And at least we can say that no fronto-orbital sulcus – the structure found in Australopithecines – appears on endocasts of earlier hominids like Homo rudolfensis (1.8 million years ago) and gay habilis (2.5 to 1.5 million years ago), though researchers are still debating whether those casts show signs of frontal opercula.
“We think another look at these specimens would be really helpful, to see if the evolution of this brain region happened in one step or if there were different stages that these older fossils could represent,” University paleoanthropologist John Hawks said. from Wisconsin. Ars.
In the gay naledi endocasts, Columbia University anthropologist Ralph Holloway and his colleagues found clear signs of frontal opercula. In casts of one skull, Brodmann’s area 47 – a part of the frontal lobe associated with processing syntax and recognizing social emotions – is particularly evident.
Does that mean gay naledi had a language?
“What exactly the extent of the communication skills would be — you can’t know just by looking at an endocast,” Holloway told Ars. But since parts of Broca’s region are so clearly visible on the casts, “it really suggests that their communication skills probably included some rudimentary language. That’s where my speculation would certainly go.”
In fact, based on endocasts belonging to other hominins from the 1.8 million years old Homo rudolfensis to the 400,000-600,000 years old Homo heidelbergensis (this includes) homo erectus at about 1.5 million years old), the capacity for at least rudimentary language in the human lineage may extend back about 1.8 million years.
But it’s still hard to say exactly how “smart” gay naledi may have been.
Hawks told us:
The best way to answer this question is to build a better picture that contains many types of evidence. We’ve debated for 150 years how smart Neanderthals were — and as scientists put together more evidence from archaeology, from pigments, from the brain, and genetics, it’s clear they weren’t all that different from us. We don’t have that much evidence yet gay nalediand that’s our challenge, to find these other sources of evidence about behavior.
What’s on your mind
The casts show that too gay nalediThe occipital lobes were larger on one side than the other – another human feature. In humans, that asymmetry is likely associated with the more developed language processing and production centers of our left hemisphere. (That asymmetry, as a side effect, may be why most of us have a dominant right hand.)
The gay naledi casts suggest asymmetry but are inconclusive. Thanks to increased language ability, this may be a common trait shared by hominids, but probably not our last common ancestor with the other great apes.
“We do know that monkeys, chimpanzees, [and] gorillas do have some dexterity, but they don’t show the same level or a more consistent pattern that you find in humanoids,” Holloway said.
Two of the skulls had larger left occipital lobes, but that doesn’t necessarily mean they gay naledi persons were right-handed. Typically in humans, a larger left occipital lobe and right frontal lobe are associated with right-handedness, but none of the gay naledi skulls retain details of the right side of the frontal lobes, which limits what we can say about handedness.
“We can only speculate about dexterity in gay naledi, unfortunately,” said Hawks. “In some other hominids, such as Neanderthals, scientists have some evidence that they were more likely to be right-handed, due to the orientation of cutting marks they sometimes accidentally made on their teeth when chewing and cutting meat. gay nalediwe have not yet found any evidence of this kind.”
The occipital lobes also provide a clue about gay naledithe visual system. The key is a structure called the lunate sulcus, which is part of the primary visual cortex. In non-human monkeys, it is larger and further forward than in humans. That’s because our visual cortex is set up differently from that of other monkeys’. The human visual cortex is generally larger, but much of that extensive area is focused on association, and the primary visual cortex eventually becomes a little smaller than in chimpanzees, our closest living relative.
That difference affects the size and position of the lunate sulcus. And in gay naledi, it’s smaller and further back – more like a human brain than a chimpanzee brain. (There are as yet no good, universally accepted endocasts of a lunate sulcus from other extinct hominids for comparison.)
First structure, then size
The presence of these human qualities in gay naledi‘s brain structure means that these traits were probably present in our last shared ancestor with the Australopithecines and may in fact be part of what it means to be a hominin.
Brain size was once widely regarded as one of the main distinguishing features of the genus HomoHolloway and his colleagues wrote in their paper. But it turns out that the same brain structures found in humans and in the gay naledi endocasts appear in several other members of the genus Homoalthough brain size varies widely from about 450 to 600 cubic cm in gay naledi up to 1450 cubic cm in modern humans. Many of those structures are more thoroughly developed in humans than in our extinct hominin relatives, but they were present at the beginning of our lineage.
That means that the development of those structures wasn’t just a result of the development of larger brains, although a larger brain size ultimately allowed us to better develop those abilities. Language, social behavior and motor planning may have been important criteria for natural selection at the time Homo branched from Australopithecusthe researchers suggest.
Where does gay naledi fit?
So gay naledi has a brain the size of an Australopithecine, but built (more or less) like that of a human. Where does it fit in the human family tree? That’s been a mystery since its discovery in 2015, when bones from 14 individuals revealed a mixture of modern and archaic qualities. The mystery got even more interesting in 2017 when uranium series put dating gay naledi on the scene around the same time as early modern man. To draw more specific conclusions about our relationship with our recently discovered cousin, paleoanthropologists need more information — and that means more fossils.
“We have so little evidence. It sounds great when you say we have this skull and that part of the skull and so on, but we really need a whole lot more. Our photo is very, very foggy,” Holloway said. He is particularly interested in a slightly earlier look at gay naledi‘s history.
“If we get more fossils that come out about 400,000 to 600,000 years ago, that would be very helpful,” Holloway said. Ars. “Unfortunately at that age you can’t do DNA analysis right now, that’s what you should really clarify [phylogenetic relationships]but I think if some more fossils come from the zone that are between 600,000 and 400,000 years old, it would be very helpful.”
And the same goes for other humanoids, of whom we know much even less than gay nalediaccording to Hawks.
“We urgently need more evidence about the skeletons of many other hominids. We know literally nothing about the bodies of Homo rudolfensis, for example,” Hawks told Ars. “Right now we know more about gay nalediNeanderthals and modern humans than any other fossil hominin, and we have to build those other monsters.”
PNAS2018. DOI: 10.1073/pnas.1720842115 (About DOIs).