Fossil records tell us that physical changes typically take 500,000 years to appear
Changes in water availability, rainfall, temperature and humidity, either globally or locally, can shape evolution. When we look at the origin and evolution of humans, many climatic changes have occurred over millions of years. Six million years ago, human ancestors separated from a branch comprising of chimpanzees and gorillas.
This new branch became known as hominin, which includes humans and the extinct humans and our immediate ancestors that could walk upright. This change corresponded to climatic changes.
We have evolved from unicellular organisms, thanks to climatic conditions. If conditions had not changed, perhaps there would have only been 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 an environment that was constant, 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 do not necessarily translate to changes in the human body or physical appearance.
Physical changes occur only 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 changes in vegetation, and that has shaped human evolution.
Climate change resulted in the most dramatic change in us: the brain size. That happened with our genus, Homo. The brain size of our ancestors belonging to the genus Australopithecus or 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 ago, 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. They needed to build tools to survive. So humans began to use their brains more, which would have affected the size of the organ.
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.
In the present day, we see genetic changes due to climate changes. These changes allow people living in high-altitude regions or colder climates to live in extreme conditions.
For example, Inuit 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 where oxygen levels are low. 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.
Tibetans have a higher capacity to retain oxygen because they live at higher altitudes where oxygen levels are low. Photo: iStock
Drastic changes can also occur, but climatic fluctuations occur at a rate of hundreds of thousands of years. 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.
New human 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. 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.
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. They 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 would perish or migrate.
Thanks to technology and modern medicine, humans have learned to survive. 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.
Rajeev Patnaik is professor at the department of geology, Panjab University
This was first published in the State of the Environment 2023 published by the Centre for Science and Environment and Down To Earth
We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.
Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.