Australia

In the current issue of Heredity, Neaves and colleagues describe the results of their analysis of 12 microsatellite loci and the mtDNA of two kangaroo species -- western and eastern grey kangaroos. The two species are sympatric across part of Australia, basically a swath through western New South Wales. Neaves and colleagues describe substantial evidence for introgression of both autosomal loci and mtDNA into both populations:

A total of 7.6% of grey kangaroos sampled from the region of sympatry displayed evidence of introgression. Although no F1 hybrids were identified, 14 M. giganteus backcrosses and 3 M. fuliginosus backcrosses were detected. In addition to introgression at nuclear microsatellite loci, a single individual also exhibited introgression of mtDNA. The two phenotypic groups apparent within the region of sympatry corresponded (in 95% of individuals) to the two clusters identified by genetic analyses. Furthermore, the two phenotypic/genetic groups within the region of sympatry corresponded to representative allopatric samples of M. giganteus and M. fuliginosus from elsewhere in the distribution. Five of the M. giganteus backcrosses identified by genetic analyses were classified as M. fuliginosus based on overall phenotype. Geographically, hybrids were located throughout the region of sympatry.

This introgression has happened between the kangaroos despite the presence of prezygotic barriers that interrupt mating even in captivity:

Physical differences in the structure of the cloacal eminence as well as the production of species-specific odours by females may allow for species recognition (Kirsch and Poole, 1972). These characteristic differences are potentially among the features that result in the unidirectional hybridization observed in captivity, with male M. giganteus frequently failing to recognize female M. fuliginosus in oestrus.

In addition, there was male sterility in captive F1 hybrids. The authors expected a unidirectional bias in introgression owing to these factors, but the evidence says that gene flow apparently has gone both directions in the wild.

Sort of interesting -- I would actually have expected there to be fewer postzygotic isolating mechanisms in marsupials because the placenta-uterus interaction isn't there complicating matters. But cases of interspecific hybridization have apparently been rarely noted -- maybe that's because Australia is small enough that phylogeographic differentiation doesn't go as far for large species. In any event, this case is another one where F1 hybrids are basically absent in the area of sympatry, yet substantial historical introgression has clearly happened. That's based on a restricted sample of 12 autosomal loci -- we would expect to see much more significant effects at a few genes if the introgressive variant had a high adaptive value.

A model for ancient humans? Well, here's a case where 12 microsatellite loci seem sufficient to document substantial historical gene flow -- whereas in the human case described last week, there are more than 600 microsatellite loci to test the hypothesis. So the human case should have more power, all things being equal.

But the humans probably don't have as simple a prior population structure. The kangaroos have two well-defined lineages with a large zone of sympatry. Ancient humans may not have been highly differentiated (given the low Neandertal-human mtDNA coalescence time, for example) and may not have had substantial zones of sympatry -- they may have been much more similar populations interacting along a narrow boundary or cline. So the phylogeography in humans will be much more subtle.

References:

Neaves LE, Zenger KR, Cooper DW, Eldridge MDB. 2010. Molecular detection of hybridization between sympatric kangaroo species in south-eastern Australia. Heredity 104:502-512. doi:10.1038/hdy.2009.137

Paul Ham reports on developments which may force the relocation of rock art in northwestern Australia:

The world’s oldest depiction of a human face could be threatened if Australian mining companies are permitted to build an explosives factory on the remote Burrup peninsula in the northwest of the country.

A bulbous image of indiscernible sex, with huge eyes and sunken cheeks, the 10,000 year-old carving is chipped out of hard rock. Thousands of other carvings, mostly of plants and animals, which date back to beyond the last Ice Age, are scattered about the peninsula.

Archeologists believe that aboriginal tribes made the distinctive carvings up to 30,000 years ago. They could be nearly twice as old as the Lascaux cave paintings in the Dordogne, France.

The West Australian paper has this report on a December 20 rally:

A rally in Perth today marked the 200th global 'stand up' for Burrup Peninsula with a renewed call for World Heritage listing for the rock art site.

Since 2006, Friends of Australian Rock Art has organised 200 vigils for the Burrup rock art in more than 35 countries and in every continent except Antarctica.

FARA spokesperson Robin Chapple said that international pressure was mounting for Australia to include the Burrup on the UNESCO World Heritage List.

Science's Michael Balter reviews the recent Cambridge conference on "Global Origins and Development of Seafaring". The article begins with a suggestion that the first inhabitants of Flores floated there on vegetation rafts by accident -- channel crossings being otherwise impossible for Lower Paleolithic hominids:

"Flores is the exception that proves the rule in terms of when seafaring really began," says Atholl Anderson, a prehistorian at the Australian National University (ANU) in Canberra. [Jon] Erlandson agrees: "Otherwise, H. erectus should have colonized Australia and the surrounding islands."

It mostly seems to be about Wallacea, Sahul, and Melanesia.

The article features a disagreement concerning the colonization of these regions. Some think that island colonizations started before seafaring technology was quite ready for prime time. In that scenario, the initial habitation of parts of Wallacea along with Australia and New Guinea was a sort of accidental chain of small founding events, possibly as early as 60,000 years ago or earlier.

The opposing viewpoint holds that these islands (and continent) were inhabited relatively late and quite suddenly, by people who had developed an advanced seafaring skill. Balter quotes University of Utah archaeologist Jim O'Connell to good effect:

In the last few years, O'Connell, together with archaeologist Jim Allen of La Trobe University in Bundoora, Australia, has argued from a detailed analysis of radiocarbon dates for a "short chronology" that puts the occupation of Sahul no earlier than about 50,000 years ago. He pointed out that by 45,000 years ago modern humans had colonized a number of islands between Sunda and Sahul, called the Wallacean Archipelago, which stretched at least 1000 kilometers even when sea levels were at their lowest. Reaching many of these islands required sea crossings of 30 to 70 kilometers, sometimes against the currents. Most animals from Asia never achieved these crossings, implying that humans must have used technology to do it. That 5000 years of colonization, O'Connell said, represented a relatively short "archaeological instant."

O'Connell also argues that some of the island sites before 40,000 years ago include deep-water fish, suggesting relatively advanced ocean-going boats at that time -- something I noted in a post on the East Timor site, Jerimalai.

Which side is right? I don't know, but it's good that they are formulating hypotheses this way, involving the technological trajectory, genetic constraints on small populations, and various ecological parameters.

References:

Balter M. 2007. In search of the world's most ancient mariners. Science 318:388-389. doi:10.1126/science.318.5849.388

Brumm and Moore (2005) review the "symbolic revolution" in the light of the Australian archaeological record.

In brief, until the mid-Holocene, there is no strong evidence in the Australian record for the systematic use of symbolism. What evidence there is consists of isolated and sporadic examples. In contrast, the mid- to late-Holocene exhibits a pattern of changes like that in Upper Paleolithic Europe, at least with respect to symbolic behavior:

As decades of archaeological research have revealed, Aboriginal social and economic systems appear to have 'intensified' and become more complex in the last 7000 years, but particularly during the middle to late Holocene (Allen and O'Connell 1995; Beaton 1985; Lourandos 1983; 1997; Lourandos and Ross 1994). Fishing technologies like rock walls, weirs, shell hooks and other equipment, as well as complex technologies for processing toxic plants appear towards the late Holocene, suggesting increased diet breadth and an intensification of marine and plant food resource extraction (Evans and Jones 1997). Along with and possibly closely related to economic intensification there appears to have been a marked increase in site usage and population density, synchronous with a growth in the size and frequency of social aggregation (Lourandos 1997). Long-distance exchange networks circulating such articles as stone artefacts, ochre and pearl shell throughout the Australian continent (see McBryde 1987) date to the middle to late Holocene, with the most extensive trading networks emerging only within the last millennium or so (Davidson et al. in press; Hiscock 1988; Tibbett 2002). Stone technology increases in complexity, a process which includes the emergence of Levallois-like stone-reduction methods around 6000 years ago (Dortch 1977; Dortch and Bordes 1977; Moore 2003a). Blade-based lithic industries and backed artefacts become well established at about the same time (Figs. 4 and 5). Ground-edge axes become widespread in the middle Holocene after a Late Pleistocene hiatus (Morwood and Hobbs 1995), and distinctive tools like large bifaces (Moore 2003b), bifacially-flaked points (Akerman and Bindon 1995), and tula adzes (Moore 2004) emerge in the Holocene (Brumm and Moore 2005:165-166).

The passage goes on to discuss changes in artistic representation, religious systems, and burial practices that also occurred in the mid-Holocene. This would seem to be the closest thing in Australia to a "revolution", but on temporal and climatic grounds, it probably is an Australian manifestation of the expansion of human population size that also occurred in pre-agricultural populations in the Old and New World alike.

The paper contrasts a "short-range" model for the evolution of behavioral modernity with a "long-range" model. In the "short-range" model, the behavioral "package" of modern humans evolved recently and quickly resulted in the dispersal of a single population across the Old World. Australians in this hypothesis must descend from this initial dispersing population, which was intensively symbol-making. Brumm and Moore consider this view problematic, since symbol-making was evidently not a central focus of Australian behavior until the Holocene:

On the other hand, if one accepts that the first colonizers were behaviourally modern -- and this is the opinion of most Australian researchers -- then the criteria used by the 'short-range' camp to identify
modern human behaviour in the Old World is undermined. The Australian record demonstrates that fully modern symbolling humans did not necessarily produce a repetitive package of symbolic traces. This in turn supports Wadley's (2001) position that a single case of symbolic storage may be sufficient for identifying modern human behaviour. The 32,000-year-old Mandu Mandu shell beads are perhaps the least ambiguous evidence for symbolic storage recovered to date from the Australian Pleistocene; they can, by themselves, confirm Davidson and Noble's (1992) contention that the first colonizers of Australia were behaviourally modern.

A pause, for dramatic effect...

Furthermore, if modern symbolic behaviour in early Australia produced a patchy archaeological record, there is no clear reason for rejecting the 'modernity' of the Middle Pleistocene record of the Old World solely on the basis of its patchy distribution.

We suggest that the Holocene Australian example could indicate that the rapid pace of change during the symbolic revolution in Africa and Europe roughly 50,000-40,000 years ago has little to do with the emergence of modern human behaviour and more to do with social, demographic, or other causes. It is possible, for example, that these changes simply reflect the reaching of an organizational threshold, that regional populations had reached a level at which new channels of information transmission became necessary to alleviate conflict and establish boundaries (Kuhn and Stiner 1998, 157) (Brumm and Moore 2005:167-168).

This also characterizes the views of Barton et al. (1994, discussed in this post) regarding the intensification of symbolic expression, and particularly art, in Upper Paleolithic Europe. There also, the real intensification of symbolic expression may not have corresponded with the arrival of modern humans, but instead apparently with demographic change and greater population densities.

References:

Brumm A, Moore MW. 2005. Symbolic revolutions and the Australian archaeological record. Camb Arch J 15:157-175. DOI link

From The Australian:

THE tiny hobbit-like humans of Indonesia may have lived in Australia before they became extinct about 11,000 years ago.

The startling claim comes from archaeologist Mike Morwood, leader of the team that in 2003 uncovered remains of the 1m-tall hominid at Liang Bua cave on Indonesia's Flores island.

...

"This is seriously being discussed now by the archaeological community in Australia as a result of our work in Indonesia," Professor Morwood said.

He suggested that further field work at sites in Indonesia and northern Australia could provide answers.

I can think of one answer... (via Gene Expression).

A few weeks ago, I posted on recent work by Clive Trueman et al. (2005) that showed a prolonged coexistence of some extinct Australian megafauna with early humans.

Today, I saw this new paper by Gifford Miller and colleagues. Here's the abstract:

Most of Australia's largest mammals became extinct 50,000 to 45,000 years ago, shortly after humans colonized the continent. Without exceptional climate change at that time, a human cause is inferred, but a mechanism remains elusive. A 140,000-year record of dietary delta13C documents a permanent reduction in food sources available to the Australian emu, beginning about the time of human colonization; a change replicated at three widely separated sites and in the marsupial wombat. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs, and nutritious grasslands to the modern fire-adapted desert scrub. Animals that could adapt survived; those that could not, became extinct.

The paper has an accompanying commentary by Christopher Johnson that explains it well:

Miller et al. studied past diets of the emu (Dromaius novaehollandiae) and an even larger flightless herbivorous bird, the extinct Genyornis newtoni (see the first figure), in the arid and semi-arid regions of the south Australian interior. By analyzing carbon isotopes in individually dated eggshells, they were able to compare the contributions of plants that use the C4 photosynthetic pathway (mainly tropical and arid-adapted grasses) and those that use the C3 pathway (most shrubs, trees, and nongrass herbs) to the diet of the birds that laid the eggs. Their collection of eggshells covers the past 140,000 years, encompassing the whole of the last glacial cycle.

Miller et al. found a sudden change in emu diet between 50,000 and 45,000 years ago. Before 50,000 years ago, emus had variable diets, with a strong contribution from C4 plants; after 45,000 years ago, they ate mostly C3 plants. Genyornis eggshells were common before 50,000 years ago, but they abruptly disappeared at the same time as the diet of the emu changed. Before then, Genyornis also ate a mixture of C3 and C4 plants, but its diet was much less variable than that of the emu through the same period, which suggests that it was a more specialized feeder (Johnson 2005:255).

This paper argues that climate change could not be responsible because there was no climate change 45,0000 years ago. Trueman et al. (2005) argued that climate change was likely responsible because the animals survived alongside humans until 30,000 years ago, when climate changes did happen. Neither the paper nor the commentary note or cite Trueman and colleagues' work. This is not necessarily a surprise, since the papers were in press at the same time, but seems like an omission.

An article by Nature news does notice the contradiction, and has this to say:

Fossils found at Cuddie Springs, New South Wales, seem to indicate that ancient fauna lived side-by-side with humans for several thousand years before finally succumbing to encroaching desertification as little as 30,000 years ago (see "Did climate shift kill off giant Australian animals?").

But Miller argues that more accurately dated fossils are needed to support this theory. "The Cuddie Springs dating remains very contentious," he says. "Most agree that the extinction event occurred between 50,000 and 45,000 years ago."

Well it is always true that better dates would help. But I think we have a lot to learn about the dynamics of ancient mass extinctions, also. Burning could well have changed the Australian habitat in ways congenial to some animals (that is, after all, why people started burning to begin with) but ultimately bad for others. But could it have wiped out the entire ecological niche of a species? Or was a reduced population ultimately rendered more susceptible to human predation? Or climate change? Or were they simply outcompeted by other animals who could handle the loss of C4 [corrected on 8/1/05] resources? Lots of questions, few answers.

References:

Johnson CN. 2005. The remaking of Australia's ecology. Science 309:255-256. Full text online

Miller GH. et al. 2005. Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction. Science 309:287-290. Full text online

Australia is well known for its unique animals. It has the most extensive diversity of marsupial mammals found anywhere in the world. Together with nearby New Guinea, it is home to the only monotreme mammals in the world, the platypus and echidna. It has not one but two species of very large flightless birds, the emu and cassowary. And until historic times, its only placental mammals were humans, dingos, and bats. So much is common knowledge.

But before the arrival of humans, the Australian fauna was even more extensive for the inclusion of many kinds of very large animals, or megafauna. These include Diprotodon, which is often compared to a rhinoceros in size and adaptive pattern, Zygomaturus, which was a somewhat smaller, bison-sized herbivore, the "marsupial lion" Thalacoleo carnifex, the giant 9-foot kangaroo Procoptodon goliah, and the large flightless bird Genyornis. A guide to the extinct Australian megafauna is available from the Museum Victoria site.

What happened to these large animals? Some archaeologists have hypothesized that the appearance of humans in Australia by 50,000 years ago may have led directly to the extinction of the megafauna. This hypothesis would be essentially like that proposed for the disappearance of the American megafauna, including mammoths, giant ground sloths, horses, and many other species in the terminal Pleistocene, called the "overkill" hypothesis. A new paper in PNAS by Clive Trueman and colleagues (2005) addresses this hypothesis by examining the archaeological evidence for megafaunal extinction.

In Australia, the evidence for the "overkill" hypothesis has mainly been derived from the coincidence of the arrival of humans and the extinction of the megafauna. As described by Trueman et al. (2005), most studies have placed the most recent evidence of megafaunal elements at around 46,500 years ago. The present evidence for human occupation of Australia places it certainly by 45,000 years ago, possibly as early as 60,000 years ago. Some archaeologists have persistently suspected that the date of occupation may be even earlier, and occasionally dates for archaeological sites have emerged to support an earlier date, only later to be questioned and retracted. In any event, there has been relatively little evidence for a substantial period of interaction of humans and megafaunal species, as would be documented by finding these species together with archaeological remains for a substantial period of time.

The current study examines one such site. From the abstract:

A number of key sites with megafauna remains that significantly postdate 46.5 ka have been excluded from consideration because of questions regarding their stratigraphic integrity. Of these sites, Cuddie Springs is the only locality in Australia where megafauna and cultural remains are found together in sequential stratigraphic horizons, dated from 36-30 ka. Verifying the stratigraphic associations found here would effectively refute the rapid-overkill model and necessitate reconsideration of the regional impacts of global climatic change on megafauna and humans in the lead up to the last glacial maximum. Here, we present geochemical evidence that demonstrates the coexistence of humans and now-extinct megafaunal species on the Australian continent for a minimum of 15 ka (Trueman et al. 2005:8381).

The chemical analysis in the paper was concentrated upon demonstrating that the bone remains actually belonged within the sediments where they were found, so that the site stratigraphy is a true record of the intercalation of human artifacts and megafaunal remains. A 1996 report at the University of Sydney Archaeology department documents the Cuddie Springs site in some detail, including the evidence for butchering of megafaunal species. So the find is not new, but its improved documentation has some substantial importance in the context of megafaunal extinction in Australia.

Here is an excerpt from the discussion:

The verification of late survival for at least some Australian megafauna has broad ramifications. Although prolonged persistence of the megafauna after human arrival certainly does not rule out a role for humans in their extinction, it does demonstrate that the extinction of the megafauna occurred over a time scale of many thousands of years. In Australia, there is no evidence for either megafaunal kill sites or contemporaneous technologies typically associated with big-game hunting (such as spear-throwers or stone-tipped projectile points). Arguments for human-mediated megafaunal extinction have commonly rested on the strength of a circumstantial case (i.e., that extinctions preceded significant climate change); therefore, humans must have been responsible. However, sites other than Cuddie Springs have yielded megafauna remains that are significantly younger than 46.5 ka, and an increasing body of evidence attests to the onset of climatic instability in Australia from ~50 ka, culminating in full glacial conditions as early as 30 ka. Climate instability is characteristic of the Late Pleistocene and is coincident with faunal extinctions on all continents (Trueman et al. 2005:8383-8384, citations elided).

In other words, climate change is very much in the game for explaining these extinctions. On the other hand, there is no real way to take humans out of the game. People were clearly hunting these animals as they expanded in population size at an unknown rate. In other regions of the world, increases in human population size were associated with increases in the breadth of prey species and apparent impacts on low-recovery rate species. Whether Australia is another example of this phenomenon will require not only a better understanding of the timing of megafaunal extinctions, but also a better modeling of ancient human population dynamics.

References:

Trueman CNG, Field JH, Dortch J, Charles B, and Wroe S. 2005. Prolonged coexistence of humans and megafauna in Pleistocene Australia. Proc Nat Acad Sci USA 102:8381-8385. PNAS online