Stick insects have lived for one million years without sex, genetic research has revealed.Scientists in Canada investigated the DNA of Timema stick insects, which live in shrubland around the west coast of the US.
They traced the ancient lineages of two species to reveal the insects' lengthy history of asexual reproduction.
The discovery could help researchers understand how life without sex is possible.
Scientists from Simon Fraser University, Canada, published their results in the journal Current Biology.Certain species of Timema stick insects were known to reproduce asexually, with females producing young in "virgin births" without the need for egg fertilisation by males.
The insects instead produce genetic clones of themselves.
Dr Tanja Schwander and her team set out to test how old these species were, and therefore to find out how long they had reproduced in this way.
By analysing the DNA of the insects, scientists were able to trace back their lineages to identify when they became a distinct species.
The team discovered that five of the asexual stick insects were "ancient", dating back more than 500,000 years. Two of them were even older.
"All the evidence points to Timema tahoe and Timema genevievae having persisted for over one million years without sex," Dr Schwander told BBC Nature.
"Our research adds to the growing amount of evidence that asexuality does not always result in the rapid extinction of a lineage," she said.
In the past, asexual reproduction has been associated with "evolutionary dead ends" because the lineages of organisms studied were often short-lived.
In more recent studies, tiny invertebrates called bdelloid rotifers and darwinulid ostracods were described as long-established asexuals by scientists investigating fossil records.
But there has been ongoing controversy surrounding these ancient asexuals. Further study suggested that asexuality was, in some cases, likely to have been a recent adaptation.
Asexual survival Dr Schwander and her team's genetic analysis confirmed that their stick insects have a long female-only history.
"Timema are indeed the oldest insects for which there is good evidence that they have been asexual for long periods of time," said Dr Schwander.
Comparing sexual and asexual species of stick insect could teach scientists more about how organisms survive without sex.
Asexuality does bring certain benefits, including rapid population growth. But the repeated cloning of genes through generations is thought to have significant negative consequences too.
This replication means that species are less able to adapt to new environments through "shuffling and tweaking" of genes.
Dr Schwander said: "Why Timema asexuals have been able to persist for so long despite all the predicted negative consequences of asexuality is the focus of ongoing studies."