Today, with rapid strides in science, it may be worthwhile to reconsider the biological importance of hybridisation, a naturally occurring and ubiquitous process, unsullied by value-laden human prejudices
(Left) The hybrid offspring sitting with its capped langur mother; (Right) the probable father (Trachypithecus phayrei) and mother (Trachypithecus pileatus) of the hybrid resting on a tree in a fragmented forest patch of northeast Bangladesh. Photo: Springer Nature
This is the third of a four-part series.
We now have evidence that way more interbreeding happened between Neanderthals and the ancestors of modern humans across Europe and Asia than scientists originally thought, a new study has found.
Scientists initially thought that interbreeding among the two groups was more isolated to a particular place and time — specifically, when they encountered each other in western Eurasia shortly after modern humans left Africa. This idea stemmed from the fact that the genomes of modern humans from outside Africa are only about 2 per cent Neanderthal, on average.
Subsequent research, however, has found that Neanderthal ancestry is 12 to 20 per cent higher in modern East Asians compared to modern Europeans.
“There’s been a lot of debate as to why East Asians seem to have a bit more Neanderthal ancestry than Europeans,” says senior study author Joshua Schraiber, a population geneticist at Temple University in Philadelphia.
“There’ve been two competing ideas. One is that East Asians happen to have interbred more with Neanderthals. The other is that, of the multiple ancestral populations of Europeans, one had very little Neanderthal ancestry, diluting the [overall] Neanderthal contribution.”
To shed light on this question of interbreeding, scientists developed computer simulations that modeled how DNA would get shared during a range of numbers of encounters between modern humans and Neanderthals. Then, they looked into which models best fit modern human genetic databases.
The researchers suggested the patterns of Neanderthal DNA inheritance seen in modern humans are best explained by not one, but multiple, independent episodes of interbreeding between Neanderthals and modern humans, first and foremost in the Middle East, but also later in both Europe and East Asia.
The dilutive effect likely also played some role in why there is less Neanderthal ancestry in modern Europeans than in modern East Asians. In other words, both multiple interbreeding episodes and dilutive effects might have occurred, contrary to what was previously thought.
This scenario of multiple episodes of interbreeding between modern humans and Neanderthals fits in with the emerging view that various human lineages had complex and frequent interactions.
For example, recent work found the mysterious human lineage known as the Denisovans apparently contributed to the modern human gene pool at least twice, leaving behind two distinct genetic components — one mostly in Papuan and Australian aboriginal populations, the other primarily in East Asian populations.
More research is needed to explain why some Neanderthal DNA was kept in the human genome and some was purged, the scientists noted. For instance, previous work suggested that evolution weeded out a great deal of Neanderthal DNA from modern human genomes.
One theory for this is that Neanderthal DNA was of less benefit to modern humans as their environments changed over time. Another theory posits that harmful mutations were more common in Neanderthals due to inbreeding.
The scientists detailed their findings online in 2018 in the November 26 issue of the journal Nature Ecology & Evolution.
Mixed-species association can result in the production of hybrid offspring in the wild.
In November, 2022, we have, to the best of our knowledge, the first observational evidence of mixed-species association between the two threatened primate species Phayre’s langur (Trachypithecus pileatus) and capped langur (Trachypithecus phayrei), in fragmented forest patches of northeast Bangladesh.
There is also a report of a presumed hybrid offspring between these species. The traditional belief held by biologists is that hybridisation, which constitutes a breakdown of species-isolation mechanisms, is essentially unnatural, and hence unwelcome.
While one can only speculate on the reasons why hybridisation was considered so abhorrent, these views were strongly subscribed to and overemphasised mainly in the early part of the last century, especially when eugenics and fascism became prominent and the purity of species was considered sacrosanct.
Today, with rapid strides in science, however, it may be worthwhile to reconsider the biological importance of hybridisation, a naturally occurring and ubiquitous process, unsullied by value-laden human prejudices.
The time has also perhaps come to conduct a dispassionate and nuanced evaluation of the much reviled and discredited big cat hybridisation experiments of the Alipore Zoo, which culminated in the birth of the litigon, Cubanacan.
Such an analysis could provide historical insights into closely held scientific beliefs on species, genetics and evolution. It could also inquire how fallible record keeping of biological data, together with preconceived scientific notions, bureaucratic science management and public attitudes, could perpetuate value-driven belief systems that are anathema to objective, value-neutral science.
It is in the wake of these developments that the hybrid big cat images of Alipore Zoo are being reproduced here for edification, and are best seen in the light of the history of science without value judgement. Indeed today, scientists are rethinking the whole concept of species based on the idea of reproductive isolation as the defining factor.
Acknowledgements : Rahul Majumdar, Dr Anindya Sinha, senior scientist, National Institute of Advanced Studies, Bangalore, Piyali Chattopadhyay Sinha, Ashish Samanta and Shibaji Bhattacharya, Alipore Zoological Gardens, Kolkata, Payel Biswas of Institute of Urban Transport, New Delhi, Subhodip Bid of National Library, Kolkata and Anirban Chaudhuri all contributed in locating the historical records of hybrid big cats in Alipore Zoo in Kolkata. The authors extend their gratitude to Dr M K Ranjitsinh and Ms Ekta Sodha, CEO of Cadmus Sodha Schools in Jamnagar for their help in confirming the record of Ranji, the tiger X lion hybrid in the zoo of His Highness, the Jamsahib Ranjitsinhji, Maharajah of Nawanagar, Gujarat, India.
Shubhobroto Ghosh is author of the Indian Zoo Inquiry and the book, ‘Dreaming In Calcutta And Channel Islands.’ He is now Project Manager of Wildlife at World Animal Protection in India
Karin Saks is a primatologist based in South Africa. Since 1997, Karin has been involved in the fostering and rehabilitation of orphan baboons, the caring of injured monkeys who have been returned to the wild and has worked towards a harmonious co-existence between these primates and humans. She has many publications on primates and has been the subject of television programmes and films
Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth
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