Becoming human – hominins

As humans, we’re curious about ourselves. Where did we come from? How did we evolve?

Hominins

Like all living things, early humans are classified using the Linnean classification system.

‘Hominin’ is a term given to humans and all of our extinct bipedal ancestors – those ancestors who walked upright on two feet. ‘Hominid’ is the term given to all modern and extinct great apes, including humans, chimpanzees, gorillas, orangutans and all their immediate ancestors.

Depending what you read, you’ll find these terms used in different ways – reminding us that scientific knowledge, including how we classify organisms, can change with new evidence. For example, DNA technology has enabled scientists to re-examine the relationships between organisms to refine the classification system.

Genetic analysis showed that chimpanzees and gorillas were more closely related to humans than they were to orang-utans. Historically all 3 had belonged in a family called Pongidae. The Pongidae family is now obsolete and gorillas, chimpanzees and orang-utans belong to the family Hominidae. Gorillas and orang-utans have their own subfamilies, while chimpanzees, humans and all of our extinct ancestors, are part of the Homininae family.

Another example of changes to primate classification due to DNA evidence is removal of the Prosimii suborder. This now defunct suborder use to include lemurs, lorises and tarsiers. Tarsiers were troublesome and were shown through DNA testing to not belong in this suborder. Today, primates are split into two major suborders: Strepsirrhini (lemurs and lorises) and Haplorhini (tarsiers, new world monkeys, old world monkeys, and apes).

Neanderthals

Neanderthals are our closest hominin relative and are the best known of our early ancestors. Some of the earliest fossil evidence of Neanderthals was found in 1856 in the Neander Valley in Germany. William King, an Irish geologist, was the first to suggest the fossils looked human, but not entirely, and so should be another species.

Several years after the Neander Valley fossils were discovered, scientists realised that other earlier fossil discoveries – from Belgium in 1829 and Gibraltar in 1848 – were also Neanderthals.

Many Neanderthal fossils and related artefacts such as stone tools have been found across Europe and down into modern day Israel, Iran and Iraq. No one knows exactly why Neanderthals went extinct and why Homo sapiens survived, although new evidence suggests a longer overlap between the species than was previously thought – see Evolutionary research – advancing our understanding of us.

Neanderthals are popularly characterised as unintelligent cavemen, but archaeological evidence showed that they used tools, buried their dead and controlled fire.

Neanderthals also looked similar to modern humans, although they were shorter and stockier with angled cheekbones, prominent brow ridges and wide noses. It is not known if Neanderthals had a language, but the size and physiology of their brains suggests it was possible. Their brain size was estimated to be between 1,200–1,900 cm3, making it larger than the brain of anatomically modern humans.

Palaeogenomics – the genetic study of early humans and other ancient populations – is further opening up our understanding of Neanderthals. For example, we now know that modern humans and Neanderthals interbred outside of Africa and that remnants of Neanderthal genes remain within all non-African humans today.

From several hominins to one

Homo floresiensis

In 2003, a joint Australian-Indonesian archaeological team unearthed the remains of a small hominin from the Liang Bua cave in Flores, Indonesia. The almost-complete skull and partial skeleton is now known as LB-1. It is the most complete Homo floresiensis fossil found to date and is the holotype for the species. Bones and teeth representing as many as 12 Homo floresiensis individuals have also been recovered at the same cave – the only site where Homo floresiensis has been found so far.

LB-1 was determined to be an adult female and a little over 1 metre tall. Her diminutive size has led to the nickname ‘hobbit’.

She had a small brain, estimated at 400 cm³, which is similar in size to a modern chimpanzee. She had reasonably large brow ridges, and her teeth were large in relation to the rest of her skull.

The supraorbital ridge or brow ridge

The brow ridge is the bony bit above the eye sockets where our eyebrows grow. Its main function is believed to be protecting the facial bones from the stresses of chewing.

Brow ridges are very pronounced in great apes like chimpanzees and gorillas. The brow ridge was one of the last traits to be lost in the evolution to anatomically modern humans. Fossils show that the brow ridge in early hominins was reduced as the cranial space grew.

What we don’t know is whether our brow ridges reduced because of what we ate or how we prepared our food or as a trade-off to allow for our bigger brain.

Some scientists have argued that LB-1 might have been an anatomically modern human with a disease or growth disorder, but most scientists now recognise Homo floresiensis as a distinct human species distinct from other hominins.

Many questions remain about this hominin. Evidence suggests they used stone tools and hunted, but it is not known if they had language or made art.

One big question for scientists right now is how Homo floresiensis is related to other species in the Homo genus. So far, no DNA has been retrieved from the bones of a Homo floresiensis individual to help answer this question.

Denisovans

One of the most recent additions to the hominin family tree is the mysterious Denisovans.

In 2008, archaeologists working in the Denisov Cave in the Alatai Mountains of Siberia excavated the phalanx (a finger bone) belonging to a 5–7-year-old girl. Relative dating suggested that the bone was around 41,000 years old!

It was also extremely exciting that scientists were able to extract DNA from the bone and sequence 99.9% of the genome. This is very unusual, because degradation usually means that only partial DNA sequencing of ancient bones is possible.

The DNA analysis revealed that the bone was distinct enough to suggest classification as a new species of archaic humans. A Denisovan genome, published in 2012, suggests the young girl had brown hair, eyes and skin.

Recent comparisons of genomes have shown interbreeding between the Denisovans and Homo sapiens (modern humans). Today, substantial levels of Denisovan ancestry are found only in the Pacific islands of Melanesia, Eastern Asia and Australia. The research also suggests genetic mutations from the Denisovans might have influenced modern human immune systems.

Genes involved in immunity would have played an important role in protecting our ancestors from new diseases as they travelled and intermingled.

As of mid-2017, only the finger bone and three teeth from Denisovans had been found. A toe bone thought to be Denisovan was later shown to be a modern human/Denisovan hybrid.

However, in 2019, research identified a 160,000-year-old partial jawbone found in 1980 to be Denisovan. This morphologically significant fossil was found in a cave on the Tibetan Plateau in Eastern Asia and is the first confirmation of Denisovans outside of Siberia. The jawbone is bigger than that found in modern humans, suggesting the Denisovans had a robust build. Genomic work had already shown that a Denisovan allele of a gene involved in adaptation to low oxygen is found in modern humans today – for example, Tibetan and Sherpa people who have traditionally lived at high altitude.

The Denisovan hominin is yet to be officially named and classified, as a more complete set of bones – a holotype – is required to name a new species. However, modern research is inching us closer to being able to fully describe these enigmatic hominins.

Ongoing research

The story of our human origins is continually unfolding and changing. Continued excavations at sites of early hominin habitation and new technological approaches are advancing our understanding and interpretation of the available evidence.

By comparing the genomes of apes, Denisovans, Neanderthals and modern humans, scientists can identify DNA segments unique to the different groups. Early results suggest modern humans underwent genetic changes involved with brain function and nervous system development, including ones involved in language development, after splitting from Neanderthals and Denisovans. Identifying and understanding these genetic changes could help explain why modern humans survived when our close relatives did not.