Are humans still evolving by Darwin's natural selection?

In 1859, Charles Darwin published On the Origin of Species, a book which transformed our understanding of how life on Earth developed - but ever since then, scientists have wondered whether humans were resourceful enough to remove themselves from the grip of natural selection.

There is no question that humans are unique in the animal world. We have developed technologies that shelter us from the harshness of the environment in a way that no other creatures have managed.

While polar bears evolved thick coats of blubber to insulate them from the Arctic cold, humans could skin that polar bear, and use the pelt as clothing to keep warm.

Does this mean that, at some point, technological advances have stopped us evolving?

Much of the story is in our genes and the sequencing of the human genome has helped unlock the answers.

By comparing the genes of people from all around the world, scientists can see how different we all are, and therefore how much we have evolved apart from each other since our species first appeared.

Skin colour is the most obvious way we have evolved apart, but there are other examples.

"We are living records of our past," says Dr Pardis Sabeti, a geneticist at Harvard University. "And so we can look at the DNA of individuals from today and get a sense of how they all came to be this way."

Another area of recent evolution is how our metabolism has changed to allow us to digest some things that we could not in the past.

The most obvious example of this is lactose, the sugar in milk. Some 10,000 years ago, before humans started farming, no one could digest this beyond a few years of age.

But today, the rate of lactose tolerance in different parts of the world is a clue to the different histories of farming across the globe. While 99% of Irish people are lactose tolerant, in South East Asia, where there is very little tradition of dairy farming, the figure is less than 5%.

So clearly our technology and inventions didn't stop us evolving in the past. But what about today?

Professor Steve Jones, a geneticist at University College London, said: "In Shakespeare's time, only about one English baby in three made it to be 21."

"All those deaths were raw material for natural selection, many of those kids died because of the genes they carried. But now, about 99% of all the babies born make it to that age."

The bulk of medical and other technological developments which protect us from our environment have come in just the past century. So in the developed world today, what is there left for natural selection to act on?

"Natural selection, if it hasn't stopped, has at least slowed down," says Jones.

But although in the developed world today, almost everyone lives long enough to pass on their genes, many of us choose not to.

Some people have three children, and some people have none, so natural selection may be working in a different way.

The realisation that people in the developed world are in effect choosing to prevent their genes from surviving beyond them has led evolutionary biologist Stephen Stearns to look at evolution in the current generations in a radical way.

A long-term medical study of a small town in Massachusetts, called Framingham, allowed him to look at the medical history of thousands of women going back to the middle of the 20th Century, and to calculate how the people that are having children differ from the population as a whole.

It has left him in no doubt that people - at least in Framingham - are still evolving and in a surprising direction.

"What we have found with height and weight basically is that natural selection appears to be operating to reduce the height and to slightly increase their weight."

This was not just a case of people eating more and there was no evidence to suggest the trend of people putting on weight and losing height would continue indefinitely.

In any case, the changes were very small and very slow, similar to those at work in Darwin's evolutionary studies.

Interestingly, Stearns believes that rather than sheltering us from natural selection, the changes that we've made to the world may actually be driving our evolution.

"We see rapid evolution when there's rapid environmental change and the biggest part of our environment is culture, and culture is exploding," says Prof Stearns.

"That's I really think the take-home message of the Framingham study, that we are continuing to evolve, that biology is going to change with the culture and it's just a matter of not being able to see it because we're stuck right in the middle of the process right now."

Technology may have limited the impact of evolutionary forces such as predation and disease, but that does not mean humans have stopped evolving.

Far from it, in a world of globalisation, rapidly advancing medical and genetic science and the increasing power of individuals to determine their own life choices, more powerful forces may come into play.

The direction of our future evolution is likely to be driven as much by us as by nature. It may be less dependent on how the world changes us, but ever more so on our growing ability to change the world.

Horizon: Are We Still Evolving? will be on BBC Two at 2100 on Tuesday 1 March 2011 or watch online afterwards via BBC iPlayer