Ian Hogg and Byron Adams peered out the windows of their helicopter as it glided over the rocky slopes of the Transantarctic Mountains, dry peaks that rise above vast ice sheets just 600 kilometers from the South Pole. Their eyes flitted across the ledges and cliffs below. It was a sunny day in January 2018, and they were searching for landmarks that matched those described in some brief notes left by a deceased entomologist who, back in 1964, had discovered an enigmatic creature in this desolate landscape. No one had seen it since.
The Transantarctic Mountains stretch more than 3,000 kilometers across the continent, from the shoreline in the north toward the interior in the south, splitting the continent in two. The mountain chain, 100 to 200 kilometers wide, acts as a dam, holding back the vast East Antarctic ice sheet, a dome that rises 3,000 meters above sea level. Glaciers fed by that ice sheet ooze through gaps between the mountain peaks and slowly empty into lower-lying West Antarctica. Dry winds screaming off the eastern plateau keep the peaks themselves largely free of ice.
In winter, temperatures in the southern Transantarctics plunge below −40 degrees Celsius. Some of the hard, thin soils on these peaks haven't tasted appreciable amounts of water for tens to hundreds of thousands of years, allowing them to accumulate caustic salts, much like the surface of Mars. Yet despite the harsh environment, a handful of tiny animals call these mountains home. Hogg and Adams had been collecting samples since 2006, trying to learn which species live where. The species that had been discovered in 1964, however—an insectlike animal called Tullbergia mediantarctica—had so far eluded them.
The location they were scanning, Mount Speed, was a low ridge in the southern Transantarctics, 700 kilometers inland from the sea. Here Shackleton Glacier pours from east to west through a gap in the mountains roughly 10 kilometers wide. Hogg, a biologist at Polar Knowledge Canada, spotted a cliff resembling one described in the entomologist's notes. The pilot landed above it, and the passengers stepped out onto a barren rock slope strewn with chunks of yellowish granite. They began to methodically peek under one rock at a time. Within minutes they found their pale beasts—dozens of white, six-legged animals smaller than sesame seeds.
The critters stepped slowly and purposefully among the sand grains, navigating with antennae that were soft and fleshy, like two outstretched fingers. The animals are extremely susceptible to dehydration, however, and within a minute of being exposed they began to shrivel and die in the dry air. Over the next few days Hogg and Adams found Tullbergia under rocks on four different slopes along the lower end of Shackleton Glacier. Sometimes the oasis they inhabited was smaller than a basketball court.
Tullbergia is one species in a larger group of springtails—primitive, wingless relatives of insects. Few people have heard of springtails, although the soil in your backyard probably harbors millions of them. These minuscule animals are found around the world—and a few species inhabit the sparse patches of ice-free ground that dot Antarctica's interior, where there is little to eat but the occasional bacterium or microscopic fungus.
How Tullbergia and other springtails got to these remote mountains, and how they survived dozens of ice ages, is a mystery that scientists are eager to solve. Since the 2018 expedition Hogg and Adams, a biologist at Brigham Young University, have been performing genetic studies on the rediscovered Tullbergia, as well as on another species of springtail they found on the same expedition. The studies, which they discussed with me and which will be published later this year, will shed new and surprising light on the history of these species, which in turn may rewrite the story of how massive ice sheets waxed and waned across the continent as ice ages came and went across millions of years. Species such as Tullbergia are also stretching our ideas about the limits of biology, reinforcing the notion that even the cruelest environments on Earth can often sustain complex animal life.
Ice Age Immigrants
Antarctica is known for its penguins and seals, but these animals live only on its coastline, fed by a rich food web of phytoplankton, fish and krill. Those iconic species cannot survive in the continent's interior, an area larger than the U.S. and Mexico combined, about 98 percent of which is blanketed in glacial ice sheets.
But starting around 1900, scientists began to find that ice-free patches of ground, kilometers in from the coast, were inhabited by animals of a different kind: tiny springtails, mites, worms and wingless flies called midges. These creatures required water and often inhabited small patches of lichens or moss on north-facing slopes, where 24-hour summer sunlight melted snow and dampened the soil. Scientists gradually found them in colder and drier places, farther inland.
In 1964 entomologist Keith Wise flew to Shackleton Glacier to see if he could find animals in one of the most secluded inland places on the continent. On December 13 he skied several kilometers up the glacier from camp until he arrived at the bottom of the Mount Speed ridge. Snowmelt trickled down a cliff, wetting the soil at its base. There Wise found two species of springtails: gray Antarctophorus subpolaris, which he had seen before in other places, and ghostly white Tullbergia, new to science.
In the decades after Wise's discovery, scientists tried to piece together a rough history of the landscape where Tullbergia was found. Seafloor sediments revealed that Antarctica had experienced 38 ice ages in the past five million years. During those freezes its glaciers thickened, rising inland and cloaking many of the mountain slopes that are exposed today. Temperatures were 5 degrees C to 10 degrees C colder than at present. Most researchers assumed the rising ice sheets “more or less wiped everything out,” says Steven Chown, a polar ecologist at Monash University in Melbourne, Australia.
Scientists reasoned that once an ice age ended, the glaciers thinned, slumping downhill and exposing more of the peaks, allowing species arriving from Patagonia, New Zealand or Australia on ocean currents or on the muddy feet of seabirds to settle anew. These immigrants would replace species that had been exterminated by the advancing glaciers. When the next ice age arrived, the newcomers would also vanish, to be replaced by another wave of immigrants after the ice retreated again. Most experts assumed that the species currently in Antarctica could not have been there for more than about 20,000 years.
Then, in 2005, came a game changer. Two different teams published genetic studies that contradicted this widespread view. Peter Convey, an ecologist at the British Antarctic Survey, teamed up with Giuliana Allegrucci of the University of Rome to compare the gene sequences of midges living in Antarctica and in Patagonia, the southern tip of South America. Based on differences in DNA sequences and basic assumptions about how quickly DNA sequences undergo random change, they estimated how long ago these species had parted ways evolutionarily. Convey admits that he expected to see a separation “in the tens of thousands of years.” But his calculations suggested that they had not mingled for 68 million years. “That was actually quite amazing,” Convey says. It meant that the Antarctic midges were not immigrants at all: instead they were descendants of the continent's original inhabitants.
Isolated for Five Million Years
Sixty-eight million years ago Antarctica was covered in lush forests, populated with dinosaurs and early mammals. It was still attached to South America, forming the last vestige of the supercontinent Gondwana, from which Africa and Australia had already separated. Only after breaking away from South America, roughly 35 million years ago, did Antarctica plunge into a deep freeze that eliminated nearly every living thing.
A second study in 2005 put the origin of some Antarctic springtails far earlier than past ice ages. Hogg and his former Ph.D. student Mark Stevens, who had worked together at the University of Waikato in New Zealand, used gene sequences to estimate when several Antarctic springtail species had diverged from species in Australia, New Zealand and Patagonia. Their results showed a separation of at least 10 million to 20 million years.
These and similar findings left many scientists at a loss to explain how tiny creatures could have persisted through so many ice ages. Some speculated that the animals might have survived in various small, isolated valleys called the McMurdo Dry Valleys in the northern section of the Transantarctic Mountains, 850 kilometers north of where Hogg and Adams had found Tullbergia. The valleys have been strangely ice-free for the past 12 million years. Others hypothesized that during the ice ages, animals might have sheltered in geothermal hotspots near a handful of volcanoes that dot the continent's coastline. And maybe after surviving each ice age in those coastal areas, they somehow traveled far inland to mountains like the ones by Shackleton Glacier.
But these ideas did not hold up to the evidence that had been collected. Tullbergia and the other animals “aren't found in other parts of Antarctica,” Adams explains. “You don't find them near the volcanoes; you don't find them on the coasts”—undercutting the idea that they inhabited those faraway places in the past.
Between 2006 and 2017 Hogg visited more than a dozen locations along the Transantarctic Mountains to collect live specimens. He and Adams, who joined some of the trips, found five species of springtails, all of them previously known. But they did not lay eyes on Tullbergia until they scoured Mount Speed in 2018.
Once Hogg brought the Tullbergia samples back to his laboratory, his team began to sequence genes from them. Ph.D. student Gemma Collins sequenced a short snippet of DNA from each creature, from a gene called cytochrome C oxidase. She spent months comparing the sequences of more than 1,100 animals found at different points along the Transantarctics (some of them collected years earlier). The comparisons would show which animals, if any, shared a common history. They would reveal whether different populations in diverse locations had been isolated from one another, perhaps by expanded ice sheets, or if they had been able to move to new territory when ice was very low.
In the warmest periods between ice ages, the West Antarctic ice sheet would have thinned and retreated. And the Ross Ice Shelf, which borders most of the central and southern mountains and floats on the sea, probably disappeared. Both events would have allowed open ocean to advance inland along the mountain chain, though not as high up on the mountains as the ice sheets had risen. Hogg speculated that during these warm stretches tiny animals could probably move around and interbreed with other previously isolated populations of the same species because broader swaths of land were ice-free. Springtails could have dispersed by floating on water. “They get into new habitat,” Hogg says, and then they manage to persist for 50,000 or 100,000 years as the ice builds upslope again.
But the results for Tullbergia and Antarctophorus suggested that even in warm times, the movement of these animals was more restricted than people thought. Two populations of Antarctophorus collected from exposed ridges on opposite sides of Shackleton Glacier appeared not to have interbred for five million years—despite the fact that they lived just 10 kilometers apart, the width of the gap that the glacier flows through. “It's quite surprising,” Hogg says. “Five million years is a long time.” It appeared that the species had not traveled at all.
Geologic evidence shows that during an especially warm period three million to five million years ago, the West Antarctic Ice Sheet collapsed multiple times. Conceivably, this would have allowed springtails to float along the mountain chain as the ocean intruded. Springtails could have crossed the 10-kilometer gap and bred with genetically different springtails there. But the Antarctophorus populations had not. The genetic results in Hogg's lab also showed that groups of Antarctophorus from Shackleton Glacier had not interbred with another population, 160 kilometers farther north along the mountains, for at least eight million years. These results suggested that even when the West Antarctic ice sheet collapsed, enough ice still remained in the Transantarctic Mountains to prevent the animals from moving around.
The analysis of Tullbergia collected around Shackleton Glacier stunned the researchers even more: the gene sequences from all four sites were virtually identical. “It's like they're all clones,” Adams says. That could mean that all the animals are descended from a couple of individuals and that these descendants have never bred with any outside populations. “That is something that we're all trying to wrestle [with] to explain,” Adams says.
Toxic Quandary
How could Tullbergia have persisted for millions of years, pinned down by ice during at least 30 ice ages, without moving more than a few kilometers or breeding with other populations? This question is all the more puzzling because for much of that time, these animals were trapped in a narrow zone between deadly ice and deadly salt.
When Hogg and Adams were helicoptering up and down Shackleton Glacier back in 2018, they often saw a faint line running across the sides of the mountains: A couple of hundred meters above the surface of the ice the rock changed color, from lighter below the line to darker above it. These “trimlines” show how high the ice rose during the last ice age—the result of subtle differences in the way that minerals oxidize when they are exposed to air rather than covered.
It is easy to imagine that as the glaciers thickened, the animals would have migrated farther up the mountainside, to stay above the ice. But there is a major problem with that explanation: the upper reaches of the mountains are loaded with toxic chemicals. Turn over a rock above the trimline at Shackleton or any other Transantarctic mountain, and the soil underneath is often crusted in white salts. “It's not a good salt. It's not Himalayan rock salt,” Adams quips. “Put your tongue on this stuff, and it will light you up.”
The salt is high in nitrate, toxic to many living things. Nitrate constantly rains down on Earth as ultraviolet radiation reacts with atmospheric gases. In most parts of the world, it does not accumulate in soils, because rain washes it away. But in dry places, like the Transantarctic Mountains, it can build up over millennia, until it reaches toxic levels. These high places also accumulate perchlorate, an oxidizing chemical used in disinfectants and rocket propellants—and famous, as discovered by the Phoenix Mars Lander, for making the surface of that planet an unpleasant place.
The salts create a catch-22 for small animals such as springtails trying to escape advancing glaciers: remaining in place means they will become buried underneath ice, but creeping uphill leads to places that are “just nasty, toxic,” Adams says. “Really crappy habitat.”
Sure enough, Hogg and Adams only found springtails living below the trimline. These places, however, would have been covered by 100 meters or more of ice at the last glacial maximum, and it would have been impossible for complex life-forms such as Tullbergia to survive in ice for tens of thousands of years. So where did the animals go?
History Rewritten
The survival of any animal depends on water, and water seems to point to an explanation for Tullbergia's unlikely endurance.
Seven hundred kilometers northwest of Shackleton Glacier the Transantarctic Mountains emerge from the interior of the continent and begin to run along the coastline. This is where the isolated McMurdo Dry Valleys are. Despite the dryness, several of the valleys hold ice-covered lakes, fed by summer meltwater. The lakes are only a few meters deep, yet high up on a few of the valley walls are bathtub rings—ancient shorelines of sand and gravel. They suggest that some of these valleys had once held hundreds of meters of water, fed by streams tumbling down the mountains. This idea is incomplete, however, because the valleys are open on their seaward ends, with nothing to hold in such deep water.
Scientists surmise that sometime during a previous ice age, the West Antarctic Ice Sheet had advanced hundreds of kilometers farther north than it currently sits, approaching the mountains and damming the mouths of the valleys at the sea, allowing big lakes to form. One of them, Glacial Lake Washburn, was at least 300 meters deep.
During the 1990s Brenda Hall, a geologist at the University of Maine, dug into the ancient sediments high up on the Lake Washburn valley wall and collected freeze-dried tatters of algal mats that had grown there. Using radiocarbon dating, she estimated that the algae—and hence the lake—had existed 23,000 to 13,000 years ago, at roughly the pinnacle of the last ice age. This finding led to a curious contention, Hall says: during the ice age, the thinking went, “the glaciers were probably melting more than they do now.”
Scientists have strained to explain how that could happen because the climate was colder. One theory is that the surrounding oceans were more widely covered in ice than they are today—leading to less evaporation and therefore fewer clouds, less snowfall and more sunlight warming the dark rocks of the mountains. This, in turn, would cause more melt high up. This increased melt could have happened along the entire length of the mountains, including where Tullbergia was found.
Closely related is a strange phenomenon that scientists now call the solid-state greenhouse effect. Most sunlight that strikes a glacier is reflected by its snowy exterior. But in the Transantarctic Mountains, where hard, dry winds slowly evaporate snow and ice, glaciers often have deep, relatively transparent ice exposed on the surface. Sunlight can penetrate a meter into this ice, warming and melting it from within. Andrew Fountain, a glaciologist at Portland State University, has found that this can occur at air temperatures down to −10 degrees C.
Hall has witnessed this phenomenon high in the southern mountains, as far as 200 kilometers south of Shackleton Glacier. “I've seen on sunny, clear days,” she says, “these films of water creeping down the front of the ice cliff.”
To Hogg and Adams, these mechanisms offer important clues into how Tullbergia and Antarctophorus, as well as small worms, mites and other animals, might have survived dozens of ice ages along the edges of glaciers such as Shackleton. Adams calls them “Goldilocks habitats”—north-facing (sun-facing) hollows with just the right configuration of dark rocks and transparent ice. Along the edge of that ice would be a narrow habitable band, maybe just a few meters wide, where slight, occasional meltwater could flush the soil of salts and also help critters rehydrate, “at least every so many years,” Adams says. As an ice age moved in, gradually pushing ice farther up the slopes, Tullbergia could have slowly moved upslope as well, maybe just a meter a year, if it was lucky enough to encounter Goldilocks habitats along the way.
These explanations sound plausible but are unfinished. Hogg and Adams, neither of whom has been back to Shackleton Glacier, need to connect the genetics to a clearer time line of how Antarctica's ice has waxed and waned. They also need to see if the pattern holds for other species. They and their students are now trying to sequence DNA from the same cytochrome gene in a species of mite and a species of nematode worm they found at Shackleton Glacier and at other locations around the southern Transantarctic Mountains. They hope that the genetic sequences will help explain how long these other animals have lived here, how they moved around in the past and how they stayed alive.
What is already apparent is that some species survived by the thinnest of margins. During glacial retreats, they could have established new outposts on nearby mountains. But with each new ice age, most of the populations died off. Tullbergia bears the scars of that brutal history in its DNA. The fact that every individual from around Shackleton Glacier carries virtually identical gene sequences suggests that at some point in the past, as few as two of the animals managed to survive. Every representative alive today is descended from those progenitors, which may have been lucky enough to be blown by a windstorm onto a patch of Goldilocks ground the size of a basketball court. Tullbergia “came extremely close to extinction,” Adams says.
Of course, entire communities of plants and animals have disappeared from Antarctica, part of the waves of extinctions that have occurred across Earth's history. Would a warmer, wetter Antarctica help Tullbergia rebound? Adams was back in the McMurdo Dry Valleys in January. Lake levels are rising, dry soils are getting moister and numbers of small animals such as certain nematode worms that live in the ground are increasing. At the same time, animals that have survived the really cold, dry, harsh soils “are decreasing in abundance, and their range across the landscape is contracting,” Adams says. Perhaps newcomers are crowding out the old hangers-on.
The question is whether Tullbergia will suffer a similar fate. “Based on what they've done in the past, my guess would be that they'd do quite well,” Adams says. “Just so long as they don't have to compete with invasive species.
Rights & Permissions
