‘Changing climatic conditions induce vegetation changes, which shapes human evolution’

Climate has played a crucial role in shaping evolution ever since life on Earth began in the ocean about 3.5 billion years ago. Microbes were the earliest forms of life, paving the way for roughly 8.7 million species today. 

Down To Earth speaks with Rajeev Patnaik, professor at the department of geology from Panjab University, to understand how climate influences human evolution and if humans could evolve into a new species. 

Rohini Krishnamurthy: What role has climate change played in shaping human evolution?

Rajeev Patnaik: Changes in water availability, rainfall, temperature and humidity, either globally or locally, can shape evolution. 

The origin and evolution of humans show many climatic changes over millions of years. Six million years ago, human ancestors separated from a branch consisting of chimpanzees and gorillas.

This new branch became known as hominin, which includes humans, extinct subspecies and our immediate ancestors that could walk upright. This change corresponded to climatic changes. 

This change corresponded to climatic changes. We have evolved from unicellular organisms thanks to climatic conditions. If conditions had not changed, perhaps we would have only seen microbes in the ocean. 

If you look at life as a tree, the branches kept changing until humans reached a particular branch. The bacteria that did not change perhaps lived in a constant environment, so they stayed the same. 

If there are gradual changes in a population and a part of it lives in a region witnessing environmental changes, it gives rise to new species. 

There are two kinds of changes: Genetic and phenotypic [observable, physical changes]. Even now, genetic changes keep happening automatically. But they don’t necessarily translate to changes in the human body or physical appearance.

Physical changes occur if there is a change in our diet or locomotion. For example, if a species ingests tough food, it will develop a heavy jaw. If it evolves to consume softer food, a heavy jaw is useless and it recedes.

Simply put, if the climate does not change, humans or any living being will not evolve. When climatic conditions change, they induce vegetation changes, which have shaped human evolution.

RK: Could you name one dramatic change in humans shaped by climate?

RP: The most dramatic change was brain size capacity. That happened with our genus, Homo. The brain size of our ancestors belonging to the genus Australopithecus, Paranthropus or Ardipithicines were ape-like. They lived in climatic conditions that did not require a big brain. Their diet and lifestyle did not need that either. 

Approximately two million years, there were dramatic climatic changes. The conditions were not conducive to human survival. Hunter-gatherers had to move around, change their diet, communicate and develop strategies to hunt prey. 

They needed to build tools to survive. So humans began to use their brains more, which would have affected their organ size. The brain capacity almost doubled from Australopithecus. 

Within our genus, Homo habilis still had lower brain capacity. But the brain capacity of Homo Erectus and Homo heidelbergensis dramatically increased. We have seen this in our fossil record.

RK: What about present-day changes in response to climate change?

RP: In the present day, we see genetic changes. These changes allow people living in high-altitude regions or colder climates to live in extreme conditions. For example, the Eskimos have evolved to thrive in colder conditions for thousands of years.

Tibetans also have a higher capacity to retain oxygen because they live at higher altitudes with low oxygen levels. 

It takes time for genetic changes to translate to changes that can be seen physically. Fossil records tell us that physical changes typically take 500,000 years to appear. Modern humans evolved somewhere around 300,000- 500,000 years ago. 

Around this time, we became distinct from Homo heidelbergensis. And it took almost a million years for heidelbergensis to separate from Homo erectus. 

Likewise, Homo neanderthalensis took hundreds of thousands of years to split from heidelbergensis. So physical changes occur gradually if climatic changes are also slow. 

Drastic changes can also occur, but climatic fluctuations occur at a rate of hundreds of thousands of years. By that standard, it would take 100,000 years for changes to show physically. 

The human body is complex and it takes millions of years for changes to occur. In contrast, insects evolve faster, where changes in size or colour can be recorded in short periods.

RK: Do you expect a new human species to arise in the future?

RP: New species can arise in the current period if a population become isolated. For example, if Inuit become isolated from civilisation and do not interact or interbreed, they may become physically distinct from the rest of the population. 

Usually, we see changes that begin with variation in a population, followed by races and sub-species. Finally, we have new species. 

In the past, we have some examples. Denisovans (who lived until 30,000 years ago from Siberia to Southeast Asia), were adapted to high altitude and low-oxygen conditions. This population gave rise to people living in high-altitude regions such as Tibet.

RK: Europe, which is a cold continent, is increasingly becoming hot and dry. What does it mean to the people living there if these events become recurrent?

RP: We will see genetic changes in Europeans if hot and dry conditions become recurrent. Around 13-14 million years ago, Europe was warm and humid. Roughly 16-18 varieties of Apes lived there. 

And then the climate changed, transforming Europe into a cooler place. All apes disappeared from the continent. Later, it became inhabited by Neanderthals, who adapted to colder climates. Much later, Homo sapiens came in. They were smart enough to adapt to changing situations. 

The climate governs every species. Homo sapiens migrated to greener pastures. Other human species died out because they could not adapt. Every species evolves in its niche. 

And if that niche becomes inhabitable due to climate change, the species will perish or migrate. Homo sapiens had all the resources to migrate long distances and adapt to changing conditions. 

If the climate in Europe continues to remain warm and humid, we may see changes in pigmentation or skin colour. Our ancestors came from Africa. Their pigmentation was brownish. 

Gradually, when people migrated to cooler conditions, people evolved lighter complexion. Cooler conditions, too, can bring about changes such as a higher accumulation of fat and changes in metabolism, and increased body hair. 

If Europeans interbreed with others, we might not see apparent changes, but if they stay isolated, we may see changes.

RK: How does the ongoing climate crisis impact human survival?

RP: Modern humans have interfered with the climate. We are to be blamed for so many drastic changes. But thanks to technology and modern medicine, humans have learned to survive. Even if something untoward were to happen due to climate fluctuations, there is every possibility that we would work around it.

Humans may not go extinct. If we look at the past, we see Australopithecus thrived for millions of years. Elephants have survived for around 2.5-3 million years. 

Modern humans have been around only a few hundred thousand years. So, unless something dramatic happens, we won’t die out. But if tomorrow, we have an asteroid impact or humans kill each other, nothing can save us.