People once lived in a vast region in north-western Australia – and it had an inland sea

21 12 2023

For much of the 65,000 years of Australia’s human history, the now-submerged northwest continental shelf connected the Kimberley and western Arnhem Land. This vast, habitable realm covered nearly 390,000 square kilometres, an area one-and-a-half times larger than New Zealand is today.

Overhead image of a coastline with modern day outlines and what it used to look like
Left: Satellite image of the submerged northwest shelf region. Right: Drowned landscape map of the study area. US Geological Survey, Geoscience Australia

It was likely a single cultural zone, with similarities in ground stone-axe technology, styles of rock art, and languages found by archaeologists in the Kimberley and Arnhem Land.

There is plenty of archaeological evidence humans once lived on continental shelves – areas that are now submerged – all around the world. Such hard evidence has been retrieved from underwater sites in the North Sea, Baltic Sea and Mediterranean Sea, and along the coasts of North and South America, South Africa and Australia.

In a newly published study in Quaternary Science Reviews, we reveal details of the complex landscape that existed on the Northwest Shelf of Australia. It was unlike any landscape found on our continent today.

A continental split

Around 18,000 years ago, the last ice age ended. Subsequent warming caused sea levels to rise and drown huge areas of the world’s continents. This process split the supercontinent of Sahul into New Guinea and Australia, and cut Tasmania off from the mainland.

Unlike in the rest of the world, the now-drowned continental shelves of Australia were thought to be environmentally unproductive and little used by First Nations peoples.

But mounting archaeological evidence shows this assumption is incorrect. Many large islands off Australia’s coast – islands that once formed part of the continental shelves – show signs of occupation before sea levels rose.

Stone tools have also recently been found on the sea floor off the coast of the Pilbara region of Western Australia.

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Rextinct: a new tool to estimate when a species went extinct

18 12 2023

If several fossils of an extinct population or species are dated, we can estimate how long ago the extinction event took place. In our new paper, we describe CRIWM, a new method to estimate extinction time using times series of fossils whose ages have been measured by radiocarbon dating. And yes, there’s an R package — Rextinct — to go with that!

While the Earth seems to gather all the conditions for life to thrive, over 99.9% of all species that ever lived are extinct today. From a distance, pristine landscapes might look similar today and millennia ago: blue seas with rocky and sandy coasts and grasslands and mountain ranges watered by rivers and lakes and covered in grass, bush and trees.

But zooming in, the picture is quite different because species identities have never stopped changing — with ‘old’ species being slowly replaced by ‘new’ ones. Fortunately, much like the collection of books in the library summarises the history of literature, the fossilised remnants of extinct organisms represent an archive of the kinds of creatures that have ever lived. This fossil record can be used to determine when and why species disappear. In that context, measuring the age of fossils is a useful task for studying the history of biodiversity and its connections to the planet’s present.

In our new paper published in the journal Quaternary Geochronology (1), we describe CRIWM (calibration-resampled inverse-weighted McInerny), a statistical method to estimate extinction time using times series of fossils that have been dated using radiocarbon dating.

Why radiocarbon dating? Easy. It is the most accurate and precise chronometric method to date fossils younger than 50,000 to 55,000 years old (2, 3). This period covers the Holocene (last 11,700 years or so), and the last stretch of the late Pleistocene (~ 130,000 years ago to the Holocene), a crucial window of time witnessing the demise of Quaternary megafauna at a planetary scale (4) (see videos herehere and here), and the global spread of anatomically modern humans (us) ‘out of Africa’ (see here and here).

Why do we need a statistical method? Fossilisation (the process of body remains being preserved in the rock record) is rare and finding a fossil is so improbable that we need maths to control for the incompleteness of the fossil record and how this fossil record relates to the period of survival of an extinct species.

A brief introduction to radiocarbon dating

First, let’s revise the basics of radiocarbon dating (also explained here and here). This chronometric technique measures the age of carbon-rich organic materials — from shells and bones to the plant and animal components used to write an ancient Koran, make a wine vintage and paint La Mona Lisa and Neanderthal caves

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