An ancient species of pint-sized humans discovered in the tropics of Indonesia may have met their demise earlier than once believed, according to an international team of scientists who re-investigated the original finding.
Published in the journal Nature this week, the group challenges reports that these inhabitants of remote Flores island co-existed with modern humans for tens of thousands of years.
They found that the youngest age for Homo floresiensis, dubbed the ‘Hobbit’, is around 50,000 years ago not between 13,000 and 11,000 years as initially claimed.
Led by Indonesian scientists and involving researchers from Griffith University’s Research Centre of Human Evolution (RCHE) the team found problems with prior dating efforts at the cave site, Liang Bua.
“In fact, Homo floresiensis seems to have disappeared soon after our species reached Flores, suggesting it was us who drove them to extinction,” says Associate Professor Maxime Aubert, a geochronologist and archaeologist at RCHE, who with RCHE’s Director Professor Rainer Grün measured the amount of uranium and thorium inside Homo floresiensis fossils to test their age.
“The science is unequivocal,’’ Aubert said.
“The youngest Hobbit skeletal remains occur at 60,000 years ago but evidence for their simple stone tools continues until 50,000 years ago. After this there are no more traces of these humans.”
While excavating at the limestone cave of Liang Bua in 2003, archaeologists found bones from diminutive humans unlike any people alive today. The researchers concluded the tiny cave dwellers evolved from an older branch of the human family that had been marooned on Flores for at least a million years. It was thought that this previously unknown population lived on Flores until about 12,000 years ago.
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Reintroduction of genetically distinct subspecies has led to hybridization in an endangered wild population
As their natural habitats continue to be destroyed, increasing numbers of displaced endangered mammals are taken to sanctuaries and rehabilitation centres worldwide. The ultimate goal of these centres is often reintroduction: to return these animals to wild populations. In a new study published today in Scientific Reports, however, Graham L Banes and Linda Vigilant of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, caution that such reintroductions can act as a form of genetic translocation. By using genetic analysis to assess a subset of historical reintroductions into Tanjung Puting National Park, Indonesia, they found that orang-utans from a non-native and genetically distinct subspecies were unwittingly released and have since hybridized with the Park’s wild population. As orang-utan subspecies are thought to have diverged around 176,000 years ago, with marked differentiation over the last 80,000 years, the researchers highlight the potential for negative effects on the viability of populations already under threat.
When Biruté Galdikas and Rod Brindamour began their pioneering orang-utan rehabilitation efforts at Camp Leakey in Tanjung Puting National Park, Central Kalimantan, all orang-utans were considered a single species. Over 14 years, from 1971 to 1985, they released at least 90 orphaned and displaced apes into the surrounding wild population. Advances in morphological and genetic studies have since revealed two species of orang-utan, however, on the islands of Borneo and Sumatra. The Bornean orang-utan is further subdivided into three distinct, geographically and reproductively isolated subspecies, which last shared a common ancestor in the Pleistocene and have differentiated substantially over tens of thousands of years.
Hybridization in the national park
Using genetic analyses and 44 years of data from Camp Leakey, Banes and Vigilant worked with Galdikas to determine the minimum extent to which she released non-native subspecies into the National Park. They found that Rani and Siswoyo, two females that Galdikas rescued from the pet trade, had originally been captured from northern West Kalimantan or Sarawak, and thus were of the non-local subspecies Pongo pygmaeus pygmaeus. Since their release into Tanjung Puting National Park, the pair have inter-bred prolifically with males of the local subspecies, Pongo pygmaeus wurmbii, producing at least 22 hybridized descendants to date. These offspring inherited a ‘cocktail’ of genes that could not normally occur in the wild.
Inter-breeding animals from two genetically distinct populations can sometimes lead to ‘hybrid vigor’, in which offspring reap the benefits of their parents’ individual qualities. This could explain how Rani came to found the biggest family of any female reintroduced at Camp Leakey, with at least 14 descendants over three generations. Though two died in infancy, the remainder are presumed to be alive and none are known to have required any veterinary interventions. However, “offspring born to parents from two genetically distinct populations, which have not been in genetic contact for significant periods of time, have also been shown to suffer poor health and reproductive success in a range of different species,” said Vigilant.
Strikingly, in stark contrast to Rani, Siswoyo had fewer surviving, healthy offspring than any other female at the site, which might be linked to such ‘outbreeding depression’. Her descendants are comparatively few, with only five first-generation and three second-generation offspring. Two of her offspring died in infancy, while infection following the latter pregnancy resulted in Siswoyo’s own death ten days after the birth. Her only daughter, Siswi, produced a stillborn offspring, a daughter that died in infancy, and a son that often needed medical interventions. Siswi herself has frequently required veterinary care, including major surgery to treat a perforated intestine.
Reintroduction only after genetic testing
“There is no definitive evidence of outbreeding depression among Bornean orang-utans,” says Banes, “but our findings are enough to cause serious alarm.” More than 1,500 orphaned and displaced orang-utans are currently awaiting release from centres on Borneo and Sumatra, which missed a deadline set forth by the Indonesian government to reintroduce all their orang-utans by the end of 2015. As their intake of displaced orang-utans increases, and as suitable habitat for reintroductions declines, there have been suggestions that they hybridize Bornean orang-utan subspecies – either in isolated, ‘mixed’ populations, or within existing wild populations.
Banes and Vigilant advocate genetic testing prior to all reintroductions of displaced animals, in accordance with established international guidelines. While their findings and recommendations apply to a broad range of endangered mammals, Banes is especially adamant that Bornean orang-utan subspecies be kept apart. “They might look roughly the same, but these orang-utans from different subpopulations haven’t shared a common ancestor for tens of thousands of years. It may be that inter-breeding them has no ill effects at all, but what if it does? Suddenly, for the sake of short-term welfare, we’ve compromised the viability of wild populations – and we can never take that back.”
Graham L. Banes, Biruté M. F. Galdikas and Linda Vigilant, Reintroduction of confiscated and displaced mammals risks outbreeding and introgression in natural populations, as evidenced by orang-utans of divergent subspecies, Scientific Reports, 25 February 2016, DOI: 10.1038/srep22026
(Text & Images’ Source: Max Planck Institute for Evolutionary Anthropology)