Did humans and their fire kill off Australia’s megafauna?

Genyornis. Courtesy of Michael Ströck & Wikimedia Commons.

Timeline, 2005: For those of us who do not live in Australia (and live instead in, say, boring old Texas), the animals that live on that continent can seem like some of the most exotic species in the world. The kangaroo, wombat, and Tasmanian devil, and most of all, the platypus, are high on the list of the unusual and bizarre in the animal kingdom.

But modern-day Australia has nothing on the Australia of 50,000 years ago when humans first arrived from Java. They encountered huge kangaroos, marsupial lions, 25-foot lizards, and tortoises the size of a subcompact car. Yet, within 5000 years, many of these animals had disappeared permanently. And since the dawn of the study of paleontology, researchers have wondered why.

Of course, it’s our fault

Of course, humans feature as the culprits in most scenarios. Just as the first people in the Americas are usually blamed at least in part for the disappearance of the American megafauna, like mammoths or giant sloths, the first people in Australia have also been suspected of hunting these animals to extinction or exposing them to diseases that decimated the populations.

As it turns out, humans may be to blame, but not through direct destruction or disease transmission. Instead, it may be the mastery of fire, the turning point in our cultural history, that ended in the extinction of many species larger than 100 pounds on the Australian continent.

Fire!

Australia’s first people probably set huge fires to signal to one another, flush animals for hunting, clear paths through what was once a mosaic of trees, shrubs, and grasses, or to encourage the growth of specific plants. The byproduct of all of this burning was catastrophic to the larger species on the continent.

The fires, according to one study, wiped out the drought-adapted plants that covered the continent’s interior, leaving behind a desert of scrub brush. The change in plant cover may have resulted in a decrease in water vapor exchange between the earth and the atmosphere with the ultimate effect of ending the yearly monsoon rains that would quench the region. Without the rains, only the hardiest, desert-ready plants survived.

You are what you eat…or ate

How could researchers possibly have elucidated these events of 45,000 years ago? By looking at fossilized bird eggs and wombat teeth. Using isotopic techniques, they assessed the types of carbon present in the bird eggs and teeth that dated back from 150,000 to 45,000 years ago. These animals genuinely were what they ate in some ways, with some isotopic markers of their diet accumulating in these tissues. Because plants metabolize different forms of carbon in different ways, the researchers could link the type of carbon isotopes they found in the egg and teeth fossils to the diet of these animals.

They found that the diet of a now-extinct species of bird, the Genyornis, consisted of the nutritious grasses of the pre-human Australian landscape. Emu eggs from before 50,000 years ago pointed to a similar diet, but eggs from 45,000 years ago indicated a shift in emu diet from nutritious grasses to the desert trees and shrubs of the current Australian interior. The vegetarian wombats also appear to have made a similar change in their diets around the same time.

Or, maybe not

And the species that are still here today, like the emu and the wombat, are the species that were general enough in their dietary needs to make the shift. The Genyornis went the way of the mammoth, possibly because its needs were too specialized for it to shift easily to a different diet. Its teeth showed no change in diet over the time period.

The researchers analyzed 1500 fossilized eggshell specimens from Genyornis and emu to solve this mystery and to pinpoint human burning practices as the culprits in the disappearance of these megafauna in a few thousand brief years. Today’s aboriginal Australians still use burning in following traditional practices, but by this time, the ecosystems have had thousands of years to adapt to burns. Thus, we don’t expect to see further dramatic disappearances of Australian fauna as a result of these practices. Indeed, some later researchers have taken issue with the idea that fire drove these changes in the first place, with some blaming hunting again, and as with many things paleontological, the precise facts of the situation remain…lost in the smoky haze of deep history.

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Placoderms had the "fun kind" of sex

Dunkleosteus, a Devonian placoderm. Pencil drawing, digital coloring, Nobu Tamura, http://www.palaeocritti.com. Obtained from Wikimedia Commons.

Timeline, 2008: From about 420 to 350 million years ago, the rulers of Earth’s seas were an unattractive-looking armored fish known today as the placoderms. This group, consisting of many species, were the bulldogs of the fish world, heavy-bodied with big ugly mouths full of protruding, potentially dangerous bony plates. Some of them were quite small, but a few species grew as large as 20 feet in length. They were the dominant vertebrate worldwide for about 70 million years.

Conventional scientific wisdom would say that these ancient fish reproduced the way modern representatives of ancient lineages do: external fertilization, the sperm fertilizing the egg with a little help from water. The wisdom was so conventional, in fact, that experts placed the rise of internal fertilization—delivery of the sperm into the female via an act of copulation—a good 200 million years after the placoderms swam the seas.

A catastrophe on the reef

In what is now Western Australia, something terrible happened about 380 million years ago in the shallow seas covering a coral reef: the oxygen that fed the reef suddenly plummeted, leaving the coral starved and unable to support the food web built around it. The outcome was a rapid, catastrophic loss of all of the species on the reef, including the placoderms. Thanks to stable plate tectonics and some good sediment coverage, these hapless animals remained preserved for the subsequent millions of years until a team of fossil hunters uncovered them. They now populate one of the most famous fossil finds in the world, the Gogo fossil sites, which are packed with perfect specimens of long-lost species.

The role of Sir David Attenborough, the world’s coolest naturalist

Among those perfect specimens—so perfect, in fact, that three-dimensional samples are available—is a species that now has the name Materpiscis attenboroughi. The name means “Attenborough’s mother fish” and requires a bit of explanation. Back in the late 1970s, Sir David Attenborough produced a wonderful nature and science series called Life on Earth. In the series, he highlighted the Gogo sites, and his interest led researchers to name the fish after him. But the first part of the name, the genus name Materpiscis, means “Mother fish.” Why? Because when this 10-inch fish died during that catastrophic reef loss, she died just before becoming a mother.

We know this because a couple of researchers working on her fossilized remains decided at the last minute to expose the fossil to one more round of acid treatment. They had pretty much decided to write her up as she was, which would have been plenty because of the preserved 3D perfection of her remains. But they agreed to that last treatment, which gently etches away layers of the fossil to reveal what lies beneath. They are glad they did, because what that last treatment exposed, inside of the adult fish, is a tiny, fossilized fish embryo, about a quarter of the size of its mother.

Eureka! Again, and again, and again

Anyone looking at that embryo, inside of that fish, might have had any number of “Eureka” thoughts in that moment. Eureka! It’s a fish embryo, 380 million years old! There aren’t that many of those lying around. But even more important, Eureka! It’s a fish embryo inside of the mother. That means that the egg was fertilized inside of the mother, where the embryo grew, nourished in her body, just as mammals do it. The embryo was even attached by a tiny, fossilized umbilical cord. A final Eureka! just might be that we can confirm the sex of this fish just based on the fact that she was pregnant when she died.

This just in: Sex is fun

The presence of an internally developing embryo in this placoderm sets the assumed evolutionary timing of internal fertilization back about 200 million years. No one would have guessed that these ancient, armored bulldog-like fish would represent the earliest-known internal fertilization. And the fact that fertilization was internal means that these animals must have copulated, the standard mechanism for getting sperm into the female to meet the egg. That recognition led one of the embryo’s discoverers to remark that this animal represents the earliest example a species engaging in “sex that was fun.”

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