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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Beautifully lifeless: the clearest ocean in the world

Thermohaline circulation

Because of the ability of UV light to shred biological molecules, researchers have speculated that life on Earth may have arisen in some dark corners, possibly under rocks or in the ocean depths where UV waves cannot reach. If that’s the case, we can rest assured that wherever life arose, it couldn’t have been a place containing the “clearest ocean waters on Earth.” That recognition today goes to a patch of water the size of the Mediterranean but lying in the middle of the Pacific, an ocean with a reputation for bounty. This area of the Pacific, however, although boasting waters of a unusual, deep violet color, is as lifeless as it is lovely.

Tough research job, but someone’s gotta do it

Researchers traveling to the area in October 2004 had been attracted to this part of the Pacific—stops included Tahiti, French Polynesia, Easter Island, and Chile—because of satellite images suggesting remarkably low chlorophyll levels in the area. This imaging allows scientists to track chlorophyll abundance in the Earth’s oceans, an important indicator of change on a major scale. When chlorophyll is low, life is scarce.

Indeed, when they traveled to the area, the researchers found some of the clearest ocean waters they had ever seen. According to one scientist on the project, the water was as clear as any of the clearest freshwater anywhere on the planet. He compared it, for example, to the clarity of the water from Lake Vanda, a pristine Antarctic lake lying buried under mountains of ice.

No chlorphyll, little life, but great clarity

But this clarity arises from a lack of chlorophyll and a lack of life. The researchers found that UV could penetrate to extraordinary depths in this area of the Pacific, as deep as 100 meters below the ocean surface. This depth sets a record for UV penetration in ocean waters, and its DNA-destroying properties carry responsibility for the dearth of life in this patch of the Pacific.

The mystery of the organic carbon

The researchers did find some evidence of a food web, however. But the organisms that live there, generally bacterial, must survive in a sort of closed system, constantly recycling the nutrients they need to live, such as nitrogen and phosphorus. Strangely, the team found that this particular part of the ocean was full of dissolved organic carbon, which is of biological origin. How an area so devoid of life could produce so much carbon derived from life remains a mystery. Researchers speculate that the bacteria are so busy addressing their nutrient needs in their limited system that they simply do not get around to degrading the carbon.

Out of the mix

Why would the Pacific harbor this clear, beautiful dead zone? The formation of this odd stretch of sea relies on several factors. The Earth’s oceans are a connected, global system of currents, water running not only at the surface but also at the depths. The thermohaline circulation, also known as the “global conveyor belt,” is one of these currents, driven by changes in the density of water in different parts of the globe. As water warms or becomes more saline, its density changes—warmer water is less dense than cold, and saltier water is more dense. As wind-driven currents move water toward the poles, the water cools and sinks when it gets to the high latitudes. This heavier water then flows back into the ocean basins and eventually wells up again. The oldest waters can take 1600 years to make this trip, but in the meantime, waters mix in the basins, making all of the Earth’s oceans pretty similar as they move their components around the planet.

The clearest ocean water on Earth, however, misses the trip. Its location in the middle of the South Pacific ensures that it doesn’t benefit from global river or ocean circulation. It never cools because it’s in the South Pacific, and thus, the water just stays put, clean and clear and almost completely lifeless.

The lovely bones: Terra preta to save our terra firma?

Charred bones set to save the world?

The Amazon river basin is home to the famous Amazonian “dark earth,” or terra preta, which recently made the news as more proof of large civilizations in the tangled Amazon forests.  This soil is renowned for its fertile properties, its loose consistency, water-holding abilities, and of course, its dark color. The people who used this earth generations ago—as many as 3000 years ago, according to one researcher—may have had no active interest in “greening” the planet, but they were very interested in getting a good crop yield for their efforts. Terra preta probably gave them just that.

Ancient farming wisdom

Somewhere along your educational path, you may have learned a few tips about farming. Rotate your crops. Let fields take a break. Till the soil. What you may not have understood as clearly were the natural processes that drove this farming wisdom.

When farmers turn over the soil, they loosen it. Earth happens to be the largest sink of carbon on terrestrial Earth, and when we move it around, some of that carbon gets released. When we try to fertilize it using dead and decomposing organic matter—compost, manure—this approach works in the short term to restore some nutrients, but microorgansims make pretty quick work of these organic remnants, returning carbon to the atmosphere again as carbon dioxide.

Thus, standard farming techniques of tilling and fertilizing and applying manure are short-lived efforts to keep the soil nutrient rich enough for planting. If nutrients are low, crop yield will be, too. And then there’s the water consideration; if the soil holds too little water or too much, that will also affect crop yield.

Magical fairy dust for crops

These factors all combine to make the terra preta soil look like magical fairy dust for crops. The soil actually is charcoal—or, in the lingo of the scientists who work with it, biochar. It is made from the rapid, pressurized burning of dead stuff—bones, tree bark—and manure. Pack it all into a metal container with a little hole for some pressure to escape, heat it to about 400 degrees Celsius, and you’ve made yourself some biochar. It apparently looks just like the charcoal you’d use at a cookout, but it has many more uses.

The carbon in the biochar is pretty inaccessible to microorganisms that would break it up, so it lasts a lot longer in the soil than your average, uncharred manure. In fact, it’s so long lasting that it’s still around in the Amazonian river basin long after the ancient farmers who used it disappeared. In addition to being a nutrient-rich and nutrient-tight source of carbon, biochar also is quite grabby with water, holding much more water than your average soil sample. That feature means that less water is required to grow crops in a biochar-laced field than would be needed in a regular, every-day kind of field.

Could soil invented in the Amazon save the Amazon?

Plants growing in the stuff do so faster, more robustly, and in greater numbers, primarily because of the rich nutrient source the biochar provides. Research indicates that the optimum combination is biochar plus fertilizer, which gives the greatest crop yield compared to either alone or neither. Using biochar could dramatically enhance global crop yields while decreasing water use and without adding a single acre of cropland. Using soil invented in the Amazon to save the Amazon rainforest has a nice “the circle is complete” aspect to it.

Although biochar has the drawback of having to be made and transported, its benefits to the planet don’t end with crop yield and water savings. The smoke generated from its preparation, in a process called pyrolysis, can be collected and used to form bio-oil, a form of renewable energy. In addition, biochar has potential as a sponge to soak up phosphates and nitrates from fertilizers before they reach our waterways, a sort of barrier against pollution. Last, this dark, magical fairy dust not only reduces carbon dioxide emissions from cropland but also significantly decreases methane and nitrous oxide emissions, both greenhouse gases that are far more potent than carbon dioxide but get considerably less press.

Termite toots causing global warming?

Termites to blame for global warming?

I was, um, in the bathroom at the Denver Zoo listening to the info feed the nice woman with the colonial accent was providing for folks using the facilities. The facts are all about poop and related activities, which I suppose is appropriate to the moment at hand. To add to the excretory atmosphere, the stall doors bear representations of animal hindquarters. Just letting you know that in case you ever want to stare at a close-up view of a baboon’s rear while you’re micturating. At any rate, as I was washing my hands, I heard a little tidbit about termites and greenhouse gases. The pleasant voice informed me that termites contribute a good percentage of the world’s greenhouse gases to the atmosphere, in the form of methane. Tooty little buggers, they must be.

It’s true. More methane than cows

Like most animals that survive on cellulose-based diets, termites have friendly micro-organisms that help them break down normally undigestible macromolecules. In the process, the micro-organisms produce a lot of methane gas. That gas, whether it’s in a cow or a termite, has to go somewhere, and that somewhere is out. Contrary to what some people may think, and according to the pleasant voice at the Denver Zoo, termites expel more of this stuff than cows do.

Should we blame the bacteria instead?

Actually, the helpful gut micro-organisms in termites are not all bacteria. Some are protozoans, depending on the termite species. But they’d be nothing without their hosts, so I guess we can just go ahead and blame them both. And I blame the Denver Zoo and their scatalogically oriented bathroom experience for the existence of this particular blog post.

Should we kill all the termites?

Well, that’s a terrible idea for any number of reasons, but as it turns out, it’s also not gonna help. One of the primary poisons used to knock of the wood-chewing insects happens also to be a “powerful greenhouse gas.” In addition, termites serve as a model for efficient harvesting of energy from biofuels, pulling about 90% from what they take in, compared to humanity’s sadly low success rates. So, yes, they eat our houses and expel about 15% of the methane in the atmosphere, but…they’re still better than we are at efficiently extracting energy from what they take in.

Why is the sky blue? Blame rocks

Early Earth’s changing landscape…and skyscape

If you traveled back in time to about 2.5 billion years ago, you wouldn’t recognize much of what you saw. The dawning, living planet back in the day sported skies of orange, shaded by an unbreathable atmosphere awash in methane gas. But through the long Proterozoic Era, those skies changed from orange to blue. Usually, we give our thanks for this change to oxygen, but a recent review traces the original actor in the color change drama to rocks. Specifically, to the phosphorus in the rocks.

Phosphorus? Really?

Phosphorus is one of the critical of elements of life. It’s a primary component of a DNA or RNA building block (nucleotide = sugar, phosphate, base). It also happens to be the primary component of the “energy currency” of cells, ATP, or adenosine triphosphate, which is really a nucleotide with three phosphates on it. Many organisms use ATP, and all organisms use phosphorus in their genetic code and their RNA.

And, a major source of it is rocks. So, yes. Phosphorus, really. Read on.

Geochemical cycles and a whole lot of gas

Several independent lines of evidence have shown that oxygen levels rose in two lengthy bursts coincident with bursts of life on Earth. The first gassy increase happened between 2.5 to 2 billion years ago. Fittingly, scientists refer to this rise in atomspheric oxygen as the Great Oxidation Event. One of the effects of the increased oxygen is that rust started showing up in the geologic record. During this Great Event, the single-celled organisms that had thrived under a presumably orange sky grew larger, and mitochondria may have arisen as a result of endosymbiosis. These cellular powerhouses are, in fact, responsible for completing energy extraction from organic molecules and using the energy to build…ATP.

The second big burst of oxygen happened about 1 billion to 540 million years ago, this time coincident with the rise of multicellular organisms and culminating in the blast of diversificiation known as the Cambrian Explosion.

What does oxygen have to do with phosphorus?

The two often hang out together as phosphate, but the real connection here is about phosphate’s contribution to life’s explosions. It may be that geologic processes, such as erosion, caused a gradual but abundant release of phosphorus from rock into the Earth’s seas. With this influx of a key component for building life, the phosphorus facilitated the early Earth equivalent of enormous algal blooms.

And guess what those algae did…and still do? Photosynthesisis. And one of the main byproducts they release from all that busy light capturing and sugar building is…oxygen. That’s the phosphorus-oxygen connection.

So, if a child ever asks you why the sky is blue and you just can’t think of the answer, you can distract them by asking them, “Did you know that the sky used to be orange?”

For your consideration

Double-membrane organelles like mitochondria are thought to have arisen through a process called endosymbiosis. What is endosymbiosis, and how could it have led to the presence of organelles like mitochondria in cells?

The algal blooms described in this paper were enormous and their influence may literally have changed the color of the sky back in the day, but the oxygen buildup and phosphorus release happened over long stretches of geological time. Today, we experience algal blooms, too. Can you identify the causes of these blooms? How do these blooms affect today’s life on Earth? Do the effects seem to be beneficial or adverse?

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