Image caption For the first time, researchers say they have identified urbanisation's signature in evolution

A "clear signal" of urbanisation has been identified in the evolution of organisms, which has implications for sustainability and human well-being.

In analysis of more than 1,600 cases around the globe, researchers said the changes could affect ecosystem services important to humans.

More than half of the world's human populations now live in urban areas, and this proportion is set to grow.

The findings appear in the Proceedings of the National Academy of Sciences.

"We found that there is a clear urban signal of phenotypic change, and also greater phenotypic change in urbanising systems compared to natural or non-urban anthropogenic systems," said co-author Marina Alberti from the University of Washington's Department of Urban Design and Planning.

"So urbanisation, globally, is clearly affecting things."

Phenotypic change refers to change in an organism's observable traits, such as it morphology, physiology, phenology, or behaviour.

Seeds of change

The changes in plants and animals included alterations in body sizes, shifts in behavioural patterns and adjustments in reproduction.

In a separate study published in 2008, researchers in France observed a rapid evolutionary change in a plant's seed size in order for it to adapt to urban life.

They found that the seeds on Crepis sancta, otherwise known as hawksbeard, were larger on specimens that lived in urban areas, when compared with the seeds from the plants growing in rural settings.

As the plant's seeds were dispersed by the wind, the researchers suggested that heavier seeds fared better because they would drop on to nearby soil, whereas the lighter seeds would be carried by the wind, resulting in them being deposited on concrete and tarmac, where it was impossible to germinate.

The speed in which this trait was expressed in the urban-dwelling plants surprised the researchers.

Professor Alberti said the changes that were observed in more than 1,600 studies were having an impact on evolution and that human activity, in the form of urbanisation, would have a lasting legacy on life on Earth.

These findings add weight to the idea that the planet is now entering an Anthropocene epoch, a geological measurement of time in which humans are having a significant global impact on the Earth's geology and ecosystems.

Entering a new age?

Prof Alberti observed: "The reason these changes are important is because they change ecosystem function, therefore they have implications for human well-being.

"This is because those changes affect, for example, biodiversity but also nutrient cycling, seed dispersal and water purification."

Prof Alberti and colleagues suggested that these changes meant that the alteration in the functions performed by the species, such as food production or the prevention of the spread of infectious diseases, would also be modified.

"There have been a lot of studies on individual cities but there had been no studies that considered the global picture to identify a global urbanisation influence on evolution," she added.

"We live on an urban planet already. This is a change that has implications for where we are heading in the future.

"We are changing the evolution of Earth and urbanisation has a role, a significant role, in that."

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