December 1, 2021

I, Science

The science magazine of Imperial College

From the origin of life through to the development of language and culture, Charlotte Mykura explores the rise of human beings

An evolutionary perspective: Who are we?Rosie Woodcock - Evolution_1024w

Of the millions of species that have ever existed on this planet, humans have singled themselves out as special. It isn’t our bizarre appearance, with our enormous heads, strange bipedal skeletons and bald physique, that immediately strikes us as our most success-endowing quality. Rather it is our immense cultural diversity. This has allowed us to accumulate a collective knowledge that gives us the quality of a single super organism. But if you take a step back to understand the 3.5 billion year story of evolution via natural selection that has given rise to you, you may well reach a greater appreciation of who we are.

From humble origins: RNA world

tRNA shutterstock_248855833 cropped
RNA has the dual property of being able to carry genetic information, and also (as with this tRNA molecule) of being able to form complex structures with catalytic properties. These functions then evolved to be carried out separately by DNA and proteins.

Life began in the form of a simple self-replicating RNA (ribonucleic acid) based molecule. The nucleic acids acquired a fatty bubble-like membrane to protect themselves, as well as coding properties and a simple energy metabolism which enabled them to thrive. Thus cellular life was born.

Later, RNA was switched for its less reactive little sister, DNA (deoxyribonucleic acid). DNA is almost exactly the same as RNA, except it is missing an oxygen atom from the ribose group.

Today there are three domains of life present on this planet. Around 3.5 billion years ago bacteria were present, followed half a billion years later by another domain of life, archea. Another billion years down the line, eukaryotes were born. Whilst bacteria and archea were to remain forever microscopic, eukaryotes evolved the capability to stick together and form large multicellular organisms such as plants and animals. We owe our existence to the humble origins of our eukaryotic ancestors.

Standing on two feet

Granted, between the origin of eukaryotes and the existence of primates, a lot of evolution happened. But now we will fast forward into the African wild of our ancestors.

Between four and seven million years ago our ancestors, of the Australopithecus genus, did something amazing. They stood up on two feet. The previously quadrupedal (four-legged) ancestor underwent a myriad of skeletal and muscular changes over the course of millions of years to allow Australopithecus to become fully erect on two legs. The spine aligned itself underneath the head; the hallux (big toe) was recruited from its thumb-like past to become part of a foot used only for walking and no longer for grasping branches. Unlike our chimpanzee-like relatives, the pelvis became bowl-shaped rather than elongated. As our legs became more substantial, we lost muscle mass from our arms and our rib-cage became slimmer and less barrel-shaped.

Bipedalism gave our ancestors the freedom to use their hands. No longer required for walking, hands could now make tools, carry young or game, gesture and hold hands. Thus walking on two legs allowed our mental and cultural evolutionary acceleration. Although Australopithecus was much smaller, much hairier with a lesser brain capacity and more restricted diet than the later genus Homo, it already had modern hands.

Two types of Australopithecus were present (sometimes referred to as different genera); the robust and the gracile. Robust australopithecines were much bulkier, with teeth geared for breaking down plant material. It was the smaller gracile lineage, with a more varied diet and greater aptitude for coping with a fluctuating environment that would eventually give rise to the Homo lineage.

Out of Africa

This image shows likely migration routes out of Africa, with numbers showing how many years ago each branch of migration occurred

During the Pleistocene epoch around 1.8 million years ago, our planet was subject to a series of climatic shifts with repeated glaciations. Homo ergaster, a large, sturdy fully bipedal ape with short arms, was roaming across Africa and Europe.

Then, between 800 and 500 thousand years ago, a more intelligent ape emerged from Homo ergaster: Homo heidelbergensis. This ape had a very large brain with a rounded cranial vault and used highly specialised projectile weapons such as wooden spears to hunt. Homo heidelbergensis gave rise to two new species, Homo neanderthalensis in Europe and Homo sapiens in Africa.

Between 900 and 300 thousand years ago, warmer periods swept across African and Eurasia, replacing desert with grasslands and the North African herds were able to move back and forth into Eurasia with the seasons. It is thought that Homo sapiens, anatomically modern humans, followed these herds for meat, and by 120 thousand years ago, tribes of individuals had left Africa for good.

Homo sapiens spread across Eurasia. Some of our ancestors followed the southern coastline down into India and across Asia, eventually reaching Australia by boats. Others were able to cross an ice sheet from Russia into Alaska, thus populating the Americas.

Today, the most diverse human populations (both genetically and anatomically) reside in Africa. As you move away from Africa, genetic diversity becomes more and more reduced. This

pattern did not occur by chance; it demonstrates the pattern of migration taken by our ancestors. It was only a small group of African humans, a thousand or so, that left Africa to populate all four corners of the globe.

The only hominin

Archaeology tells us that the Neanderthals were pushed further and further into Western Europe, disappearing altogether around 29 thousand years ago. Whether this extinction was due to conflict with Homo sapiens, competition or changes in climate is a topic of fierce debate among evolutionary biologists.

Curiously, sequencing of the Neanderthal genome in 2010 taught us that 1-4% of non- African human DNA has Neanderthal origin. No humans of pure African descent contain Neanderthal DNA, since their ancestors never came into contact with Neanderthals (who are thought to have exclusively populated Eurasia). At some point in our history, Homo sapiens and Homo neanderthalensis interbred in Europe.

Language, art, culture

Chauvet Cave_Thomas_T  5602930382_cee5f3f43a_bWhat set us apart from our hominin relatives? They were just as fast and as strong as us and in some cases even had larger brains.

For example, the gene FOXP2, thought to have arisen between 200 and 50 thousand years ago in the human lineage, allows fine motor and neural control of the mouth and tongue; allowing the existence of complex speech.

Humans brought with them an immense toolkit to Europe; cave art, figurines, symbolism, language, religion. There was no one point when humans become modern, but a gradient of cultural evolution that started within Africa 300 thousand years ago put us in greater stead to out-compete our Homo counterparts in a changing environment.

Between 8,500 and 2,500 years ago, humans started abandoning their hunter-gatherer way of life for one in which they had fixed homes and farmed the land: thus society was born.

Homo sapiens: where next?

As directional natural selection is relaxed since our cumulative culture provides entirely for our needs, we have lost the drive to retain the ability to survive in a harsh environment.

As we more and more extensively define our artificial environment, we adapt to that environment which our recent ancestors have created. So where next? Will Homo sapiens reach a greater level of collective intelligence, save their planet and learn to thrive? Or consume resources to their ultimate demise? Only time will tell.

Charlotte Mykura is a second year PhD student studying epigenetics

Images: Top/featured image: Rosie Woodcock; tRNA by Petarg (Shutterstock); Migration map, Wikimedia Commons; Cave art: Etologic horse study, Chauvet´s cave by Thomas T (Flickr; Creative Commons)