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Our most ancient ancestors were battling climate change to survive droughts, freezing and flooding.

And now a growing number of scientists believe that shifts in the Earth's climate are responsible for creating some of our most distinctive characteristics.

Their research suggests that large evolutionary leaps, such as the development of our bigger brains and ability to create and use complex tools, coincided with periods of volatile climate change. 

A growing number of scientists believe that shifts in the Earth's climate are responsible for creating some of human's most distinctive characteristics such as our big brains and ability to develop tools. This conceptual image shows four stages of human evolution: Australopithecus, Home habilis, Homo erectus and Homo sapien

Peter deMenocal, of Columbia University, admitted to Smithsonian that the scientific theory is currently based on 'coincidence' but experts are examining ancient teeth and seafloor sediments to support the idea.  

The data could help scientists unravel just how climate may have sparked dramatic advances in our evolution and seems to support the idea that big leaps were not driven by our adaptation to one environmental change, but to a series of changes as the environment alternated between wet and dry. 

Rick Potts, of the Smithsonian Institution's Human Origins Programme, said that at least two major evolutionary 'leaps' can be linked to climate change.

They happened around three million years ago when Australopithecus afarensis  - known as Lucy - became extinct and Homo appeared, with fossils revealing the evolution of a larger brain and the very first tools. 

The data could help scientists unravel just how climate may have sparked dramatic advances in our evolution and seems to support the idea that big leaps were not driven by our adaptation to one environmental change, but to a series of changes as the environment alternated between wet and dry. A drought is pictured

The other example cited is between two and 1.5 million years ago, when Homo erectus used sophisticated bladed axes, leading to early humans leaving Africa.

Dr Potts says that both these events coincide with dramatic shifts in climate when wooded areas turned to grassland as the temperature warmed.

He suggests that the change in environment three million years ago led to humans walking upright instead of climbing.

Dr deMenocal said samples taken from the sea floor along the coast of Africa show different layers of sediment that are darker in times of drought and light in wet periods.

The cores show that every 20,000 years the climate swapped from wet to dry and the effect was very pronounced during the two important periods of early human evolution.

Mark Maslin of University College London (UCL) said further evidence can be found in the Great Rift Valley, which runs through Ethiopia, Kenya and Tanzania.

By studying the rock record, he found that lake basins were very sensitive to changes in climate approximately 2.7 million and 1.5 million years ago.

Three million years ago Australopithecus afarensis, known as Lucy (a skeleton is pictured left and copy of the skull, right) became extinct and Homo appeared, with fossils revealing the evolution of a larger brain and the very first tools. One expert says the evolutionary leap coincides with 'pulses' in climate change

Dr Maslin said: 'We found that these particular periods, or "pulses" when the lakes come and go correlate directly with major changes in human evolution. The two records are absolutely compatible.'

Scientists have also looked at our ancestors' fossilised teeth to understand how their diet changed according with the climate.

They found that early humans ate a varied diet, indicating they became flexible to cope with a changing environment.

However, the research does not explain why a shifting climate may have triggered evolutionary leaps.

Dr Maslin acknowledged any one of many theories could be correct, including the idea that humans had to develop larger brains to work out how to feed themselves in times of drought when nutrients were scarce.

He also said that in wet periods when lots of resources were available, females may have been more careful to select the most successful men as partners, leading to smarter and stronger children.

John Stewart of Bournemouth University, thinks people moved around the land responding to environmental changes so that populations became isolated and evolved quickly in pockets of the land. 

Dr Mark Maslin said that rock samples from the Great Rift Valley, which runs through Ethiopia, Kenya and Tanzania indicate that lake basins were very sensitive to changes in climate approximately 2.7 million and 1.5 million years ago, when a burst of human evolution took place. The lakes still disappear in times of drought and fill up with heavy rainfall today. Here, hippos relax in one of the lakes in Kenya