Australia: The Land Where Time Began

A biography of the Australian continent 

Did the Denisovans Cross Wallace's Line?

Genetic evidence has been found of the hybridisation of Denisovans (Krause et al., 2010; Reich et al., 2010) with populations of modern humans in Island Southeast Asia, Australia, and the Pacific (Reich et al., 2011), a situation that was completely unexpected. The Denisovan genome (Meyer et al., 2012) reference specimen, from a young girl's distal phalanx, was recovered from Denisova Cave that is geographically distant, in the Altai Mountains of Russia. 3 mitochondrial genomes from material in the cave, that was dated by associated faunas (Gibbons, 2011) that have been poorly dated to more then 50,000 BP. The Denisovan population had a larger overall size, in the long term, than the Neanderthals (Pääbo, 2013; Pennisi, 2013) which suggests the Denisovans were previously widespread across the East Asian mainland. Interbreeding with modern humans appears to have occurred only in Island Southeast Asia, and such a situation would require marine crossings, which would raise questions about the Denisovan distribution and fossil record in Island Southeast Asia.

The distribution of modern populations that contain detectable amounts of Denisovan DNA is unexpected as there are no known such populations in mainland Asia. Introgressed DNA referred to a small amounts of DNA from a species that is found in the DNA from another species. Islands to the east of Wallace's Line are the only places where Denisovan DNA has been found. The isolated Aboriginal populations of New Guinea and Australia, with about 3-4 % (Meyer et al., 2012), are the populations of  modern humans, though lower percentages have been found in a range of populations in Island Southeast Asia. Early Southeast Asian hunter-gatherers and later Neolithic farmers (Reich et al., 2012) are believed to be ancestral to groups in this area,

The border between ecosystems dominated by placental mammals to the west, Wallace's Line (Huxley, 1868), one of the biggest biogeographic disjunctions in the world, and Lydekker's Line, that is less well-known, the border that has the ecosystems dominated by marsupials to the east, with only 2 terrestrial mammals known to have crossed it, anatomically modern humans and rats. Wallacea is a zone of biological transition situated between Wallace's Line and Lydekker's Line, and an area that needs to be crossed by any animals migrating between Asia and Australia. A separate dispersal across Wallace's Line is indicated by the discovery in 2003 of the "Hobbit", Homo floresiensis (Morwood &Jungers, 2009), while a foot bone from Callao in the Philippines, from about 67,000 BP represents a small-bodied hominin, the taxonomic affiliation of which is unknown (Stewart & Stringer, 2010). It is suggested that other hominin species had the capacity to cross the powerful ocean currents that formed and maintain Wallace's Line, even at times of low sea level, though these taxa remain enigmatic.

The diverse ecological range of Denisovans appears to have covered mainland Asia and Island Southeast Asia. The authors1 suggest the large historical population size that is inferred is consistent with the use of extensive regions of savannah on the Sunda Shelf that was exposed as a refugium during glacial phases of the Pleistocene (Stewart & Stringer, 2012). During climatic cycles the exposed shelf would have allowed migration , northwards and southwards.

The Denisovan DNA gene flow into modern human populations somewhere on the Asian mainland might have been suggested by the location of the Denisovan reference specimens in the Altai Mountains, before it spread throughout the Southeast Asian region. Overwriting by the DNA of incoming East Asian populations in areas other than Island Southeast Asia is suggested by the authors1 to be possibly the easiest explanation of the lack of Denisovan introgression in current mainland populations. No evidence was found, however, by analysis of the indigenous negrito/hunter-gatherer populations of Malaysia and the Andaman Islands, which revealed there was no introgression of Denisovan DNA, in spite of the Andaman Islanders being isolated for a long period of time, show no admixture with other populations of East Asia (Reich et al., 2011). Genomic analysis of an ancient modern human from China, Tianyuan, about 40,000 BP, did not detect any Denisovan DNA (Fu et al., 2013), which the authors1 suggest argues against the existence in prehistoric interbreeding signal that has been overwritten. 

According to the authors1 these observations, together, argue for no introgression of Denisovan DNA on the Asian mainland. The source of Denisovan gene flow appears to have been to the east of Wallace's Line, with the missing Denisovan DNA in mainland populations being explained by the limitation of the reverse dispersal of introgressed populations by Wallace's Line. The Denisovan-introgressed populations outside of Australia and New Guinea appeared to have been diluted by subsequent movement of East Asian/Neolithic modern humans, and also carried the signal further throughout the area and across the Pacific (Reich at al., 2011). Homo floresiensis is the only hominin that has been well characterised to have crossed Wallace's Line prior to modern humans, their affinities still remaining enigmatic, though it is suggested by morphological analyses of their remains that they derived from an early Homo erectus, or possibly even more primitive species (Morwood & Jungers, 2009). An early presence is supported by a stone tool record on Flores that dated to more than 1 Ma ((Morwood & Jungers, 2009). Trying to identify Homo floresiensis as a regional representative of Denisovans, in spite of its presence beyond Wallace's Line, is difficult to reconcile with the enlarged molars of the Denisovans, and the divergence date of modern human populations from Denisovans which has been estimated by using DNA at about 1 Ma, or 170,000-700,000 years ago with genomic data (Meyer et al., 2012).

According to recent reports the Denisovan genome contains large amounts of introgressed Neanderthal genomic DNA (Pääbo, 2013; Pennisi, 2013), which has been suggested to possibly relate to the considerable differences between these estimates of the divergence dates. Estimates of both phylogenetic relationships and genomic divergence dates between Neanderthals, Denisovans, and modern humans will be affected by this. An alternative suggestion is that the older mitochondrial divergence date of about 1 Ma may reflect the input of more Ancient Asian populations or possibly all the dates are overestimates that result from the temporal dependency of molecular rates, and the erroneous low rate that is produced by the distant chimp-human external calibration (Ho et al., 2011).

The authors1 have therefore inferred that Homo floresiensis was an endemic species with a lineage that originated at least 1 Ma, that was restricted to a small region of Wallacea, while the Denisovans probably arrived during the mid-Pleistocene, some time after 600,000 years ago, then spread more widely in the region. To the east of the line the Denisovans may be represented by the Callao specimen in the Philippines, or possibly have not yet been recognised. In Asia, other enigmatic hominin remains - from Narmada (India) and Dali, Jinniushan, Maba, and Xujiayao (China) - may represent the Denisovan population that was apparently previously more extensive, or possibly yet other species.

It has also been reported that the Denisovan genome also contains a small contribution from another archaic population that is unknown at the present (Pääbo, 2013; Pennisi, 2013). A question is whether the Denisovans interbred with a more ancient species, such as H. erectus, H. antecessor, or possibly a H. heidelbergensis in Asia (Stringer, 2012).  The genomic divergence may be compatible with a recent model, given the uncertainties of the molecular dates, that suggests modern humans, Neanderthals and Denisovans are a trichotomy that originated from H. heidelbergensis, a species that was widely dispersed in the Middle Pleistocene, possibly about 400,000 BP (Stringer, 2012). Tantalising glimpses of a diversity of hominin groups is all that can be obtained from the fragmentary and disparate fossil record of East Asia. Across Wallacea the apparently widespread distribution of early hominins, exemplified by the discoveries on Flores and in the Philippines, raises the possibility that they may have extended to the Sahul Shelf, as well as regions such as Australia and New Guinea. There is a question as to why the gene flow between Denisovans and modern human populations occur primarily mainly east of the Wallace Line and not the mainland of Asia. The first groups of modern humans to encounter the established Denisovan populations were likely to have been of very limited size, given that the intentional dispersal to Wallacea would require the use of watercraft. Either interbreeding may be more likely under these circumstances, or any interbreeding occurring is more likely to be preserved as a signal in descendants. Gene flow from the Denisovans is suggested by genomic evidence to have been largely male-mediated, providing some clues regarding the nature of the interactions (Meyer et al., 2012). Additionally, rapid dispersal of modern humans into Wallacea is likely to have subjected them to a wide range of pathogens they had previously not been exposed to, with the result that alleles for disease resistance that were obtained by hybridisation with native populations may have had selective advantages (Abi-Rached et al., 2011). The first group of modern humans to leave Africa, also probably of limited size, similarly appear to have interbred with the established the Neanderthal populations they initially encountered in western Asia (Green et al., 2012). According to the authors1 an anticipated wealth of new genomic data are set to illuminate still more the nature of interactions between modern humans and Neanderthals and Denisovans, as well as the extent and the possible functionality of the DNA exchanged.

Sources & Further reading

  1. Cooper, A. and C. B. Stringer (2013). "Did the Denisovans Cross Wallace's Line?" Science 342(6156): 321-323.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated: 20/10/2013
Home
Journey Back Through Time
Geology
Biology
     Fauna
     Flora
Climate
Hydrology
Environment
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading