Yale University research team, studied the sexual activity of more than 1,500 different animal species to discover the pattern of sexual preference.
These ethologists study animals including crabs, snakes, cows and monkeys.
The researchers according to the statement said they were puzzled by why homosexuality persists “despite not having an obvious evolutionary benefit”. Conception is only possible through heterosexuality (copulation between male and female).
Yet, the researchers noted the sexual preference becoming widespread.
Surprisingly, the researchers believe that rather than evolving after heterosexuality as previously thought, homosexual behaviour was part of our earliest DNA.
“If any trait other than homosexuality had been observed in such a diverse array of species it would be widely accepted as being part of our ancestral DNA rather than something that evolved later,” says Julia Monk, the first author on the paper.
The historical suggestion of “heterosexuality as the norm for society” has prevented the idea of us evolving as bisexual from being considered in the past, Ms Monk said.
For many years, this “Darwinian paradox” remains an enigma for scientists. Why do animals practice same-sex sexual activity when it has no obvious evolutionary benefit and could lead to extinction if all members of species practice it?
In response to this, the Yale researchers think they now have the answer. They suggest that our earliest animal ancestors practised “indiscriminate sexual behaviours directed towards all sexes”, rather than purely heterosexual sex.
“By shifting the lens through which we study animal sexual behaviour, we can more fruitfully examine the evolutionary history of diverse sexual strategies,” says Ms Monk.
As part of their study into same-sex sexual behaviour, the team examined anything that would not directly result in offspring.
This included those directed at individuals of the same sex, different species, dead bodies, inanimate objects and self-stimulation.
Earlier theories into why homosexual behaviour has persisted through evolution have suggested it is due to mistaken identity, the prison effect, dominance and intrasexual conflict.
Ms Monk and her team suggest that while various processes have shaped the persistence of same-sex sexual behaviour, there is no need to explain its origins as it is innate ― always in the animals.
They say that the heterosexual behaviour is likely a “derived trait”‘ which arose out of the bisexual nature of ancestor species along with homosexual behaviour.
Tracing the evolution of the trait, the researchers said it started with the earliest multicellular immobile species.
The team say the evidence for their theory comes in part from echinoderms.
The name echinoderm is a phylum classification of organisms like starfish, brittle stars, sea cucumbers, sea urchins, sand dollars, and crinoids species which show bisexual behaviours.
Those species bear a close resemblance to the ancestral organisms in which sexual behaviours evolved.
As sexual behaviour in species is more widely studied homosexual and bisexual behaviours are likely going to be more common than species that purely practice heterosexual behaviours, researchers claim.
Homosexuality in nature appears counter-intuitive but is observed in a range of species around the world.
There has yet to be an accepted explanation based on neurological, chemical or behavioural factors to explain why some animals are homosexual and some or heterosexual.