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Scientists have unraveled the origins of human pregnancy by tracing how our early mammal ancestors first evolved to give birth to live young.

They found rogue fragments of DNA that jumped around the genome millions of years ago caused switched off the processes needed to lay eggs.

Instead, thousands of genes used in other organs moved and were turned on in the uterus to allow a foetus to develop, effectively rewiring our ancient mammalian ancestors' biology.

This diagram shows where the 2,502 genetic changes occurred that led mammals to give birth to live young

In total they found that there were 2,502 genes added to the active genome in the uterus.

Their findings help to explain how and when mammals first developed a way of allowing their offspring to develop in the womb before birth, an evolutionary innovation that helped them to dominate the planet and has allowed humans to develop such big brains.

Dr Vincent Lynch, an evolutionary geneticist at the University of Chicago, said: 'Our results indicate that a major mechanism in the origin of pregnancy was the recruitment of genes that were ancestrally expressed in other organ and tissue systems.'

The researchers, whose work is published in the journal Cell Reports, compared the geneomes of pregnant mammals such as dogs and pigs with those of egg laying animals.

While humans might be separated from animals such as frogs and chickens by millions of years of evolution, one of the most surprising findings from projects like the human genome project was that we share much of our DNA with these other animals.

For example humans share 98.5 per cent of their genome with chimpanzees, while about 75 per cent of our DNA is identical to chickens.

This surprising level of genetic similarity is explained because not all genes are switched on at all times.

Instead they are either completely inactivated in some species or selectively turned on in different tissues, where they perform differing roles.

Using a technique known as RNA sequencing, the researchers behind the latest study were able to identify which genes were active and which had been switched off in the uteruses of 12 different animals.

These included dogs, cows, horses, pigs and armadillos - all mammals that develop fetuses in their wombs. They also examined the genes from short tailed opossums, which are marsupials, and an ancestor of ancient egg laying mammals, the platypus.

They also looked at genes from a lizard uterus, chicken uterus and frog uterus.

This allowed the researchers to trace when in evolutionary history these genes were probably switched on or off.

The relatively long time human babies spend developing inside their mother's wombs helped our species to dominate the planet by offering protection while our brains and complex nervous systems develop

Duck-billed platypus, pictured above, are one of just five remaining species of mammal that still lay eggs

Reptiles lay leathery eggs, like the Komodo dragon egg above, where their young develop until hatching

They found that thousands of short scraps of DNA that now regulate the hormonal changes of cells in the lining of the uterus, or endometrium, during pregnancy had once been ancient genes that jumped around the genome.

Dr Lynch said these genes - called transposable elements - delivered new functions to the endometrium such as controlling the immune system and sending signals between the mother's body and the fetus.

They calculated that there were 500 genes switched on or that invaded the genome and 296 turned off when mammals first diverged from their common ancestor with reptiles and birds.

A further 1,167 extra genes appeared and 239 turned off when live young baring mammals evolved from early egg laying mammals - the ancestors of the platypus.

Birds deposit a hard shell around their eggs, like those pictured above the black backed gull, as they move through their ovary ducts and uterus before laying, which helps to protect the embryos inside until they hatch

Amphibians like this common frog lay huge numbers of eggs in spawn as they do not care for their young

They then found that a further 835 genes were turned on and 185 were turned off when our own branch of the mammalian evolutionary tree split - the placental mammals - split from marsupials.

Dr Lynch said: 'For the first time, we have a good understanding of how something completely novel evolves in nature, of how this new way of reproducing came to be.

'We found lots of genes important for maintaining hormone signaling and mediating maternal-fetal communication, which are essential for pregnancy, evolved to be expressed in the uterus in early mammals.

'But immune suppression genes stand out. The fetus is genetically distinct from the mother - if these immune genes weren't expressed in the uterus, the fetus would be recognized by the mother's immune system as foreign and attacked like any other parasite.

'Genes need some way of knowing when and where to be expressed.

'Transposable elements appear to have brought this information, allowing old genes to be expressed in a new location, the uterus, during pregnancy.

'Mammals very likely have a progesterone-responsive uterus because of these transposons.'