Modern mammals are known for their big brains. But new analyses of mammal skulls from creatures that lived shortly after the dinosaur mass extinction show that those brains weren’t always a foregone conclusion. For at least 10 million years after the dinosaurs disappeared, mammals got a lot brawnier but not brainier, researchers report in the April 1 Science.
That bucks conventional wisdom, to put it mildly. “I thought, it’s not possible, there must be something that I did wrong,” says Ornella Bertrand, a mammal paleontologist at the University of Edinburgh. “It really threw me off. How am I going to explain that they were not smart?”
Modern mammals have the largest brains in the animal kingdom relative to their body size. How and when that brain evolution happened is a mystery. One idea has been that the disappearance of all nonbird dinosaurs following an asteroid impact at the end of the Mesozoic Era 66 million years ago left a vacuum for mammals to fill (SN: 1/25/17). Recent discoveries of fossils dating to the Paleocene — the immediately post-extinction epoch spanning 66 million to 56 million years ago — does reveal a flourishing menagerie of weird and wonderful mammal species, many much bigger than their Mesozoic predecessors (SN: 10/24/19). It was the dawn of the Age of Mammals. Before those fossil finds, the prevailing wisdom was that in the wake of the mass dino extinction, mammals’ brains most likely grew apace with their bodies, everything increasing together like an expanding balloon, Bertrand says. But those discoveries of Paleocene fossil troves in Colorado and New Mexico, as well as reexaminations of fossils previously found in France, are now unraveling that story, by offering scientists the chance to actually measure the size of mammals’ brains over time.
Bertrand and her colleagues used CT scanning to create 3-D images of the skulls of different types of ancient mammals from both before and after the extinction event. Those specimens included mammals from 17 groups dating to the Paleocene and 17 to the Eocene, the epoch that spanned 56 million to 34 million years ago.
What the team found was a shock: Relative to their body sizes, Paleocene mammal brains were relatively smaller than those of Mesozoic mammals. It wasn’t until the Eocene that mammal brains began to grow, particularly in certain sensory regions, the team reports.
To assess how the sizes and shapes of those sensory regions also changed over time, Bertrand looked for the edges of different parts of the brains within the 3-D skull models, tracing them like a sculptor working with clay. The size of mammals’ olfactory bulbs, responsible for sense of smell, didn’t change over time, the researchers found — and that makes sense, because even Mesozoic mammals were good sniffers, she says.
The really big brain changes were to come in the neocortex, which is responsible for visual processing, memory and motor control, among other skills. But those kinds of changes are metabolically costly, Bertrand says. “To have a big brain, you need to sleep and eat, and if you don’t do that you get cranky, and your brain just doesn’t function.” So, the team proposes, as the world shook off the dust of the mass extinction, brawn was the priority for mammals, helping them swiftly spread out into newly available ecological niches. But after 10 million years or so, the metabolic calculations had changed, and competition within those niches was ramping up. As a result, mammals began to develop new sets of skills that could help them snag hard-to-reach fruit from a branch, escape a predator or catch prey.
Other factors — such as social behavior or parental care — have been important to the overall evolution of mammals’ big brains. But these new finds suggest that, at least at the dawn of the Age of Mammals, ecology — and competition between species — gave a big push to brain evolution, wrote biologist Felisa Smith of the University of New Mexico in Albuquerque in a commentary in the same issue of Science. “An exciting aspect of these findings is that they raise a new question: Why did large brains evolve independently and concurrently in many mammal groups?” says evolutionary biologist David Grossnickle of the University of Washington in Seattle.
Most modern mammals have relatively large brains, so studies that examine only modern species might conclude that large brains evolved once in mammal ancestors, Grossnickle says. But what this study uncovered is a “much more interesting and nuanced story,” that these brains evolved separately in many different groups, he says. And that shows just how important fossils can be to stitching together an accurate tapestry of evolutionary history.
Researchers have finally deciphered a complete human genetic instruction book from cover to cover.
The completion of the human genome has been announced a couple of times in the past, but those were actually incomplete drafts. “We really mean it this time,” says Evan Eichler, a human geneticist and Howard Hughes Medical Institute investigator at the University of Washington in Seattle.
The completed genome is presented in a series of papers published online March 31 in Science and Nature Methods.
An international team of researchers, including Eichler, used new DNA sequencing technology to untangle repetitive stretches of DNA that were redacted from an earlier version of the genome, widely used as a reference for guiding biomedical research.
Deciphering those tricky stretches adds about 200 million DNA bases, about 8 percent of the genome, to the instruction book, researchers report in Science. That’s essentially an entire chapter. And it’s a juicy one, containing the first-ever looks at the short arms of some chromosomes, long-lost genes and important parts of chromosomes called centromeres — where machinery responsible for divvying up DNA grips the chromosome.
“Some of the regions that were missing actually turn out to be the most interesting,” says Rajiv McCoy, a human geneticist at Johns Hopkins University, who was part of the team known as the Telomere-to-Telomere (T2T) Consortium assembling the complete genome. “It’s exciting because we get to take the first look inside these regions and see what we can find.” Telomeres are repetitive stretches of DNA found at the ends of chromosomes. Like aglets on shoelaces, they may help keep chromosomes from unraveling.
Data from the effort are already available for other researchers to explore. And some, like geneticist Ting Wang of Washington University School of Medicine in St. Louis, have already delved in. “Having a complete genome reference definitely improves biomedical studies.… It’s an extremely useful resource,” he says. “There’s no question that this is an important achievement.”
But, Wang says, “the human genome isn’t quite complete yet.”
To understand why and what this new volume of the human genetic encyclopedia tells us, here’s a closer look at the milestone. What did the researchers do? Eichler is careful to point out that “this is the completion of a human genome. There is no such thing as the human genome.” Any two people will have large portions of their genomes that range from very similar to virtually identical and “smaller portions that are wildly different.” A reference genome can help researchers see where people differ, which can point to genes that may be involved in diseases. Having a view of the entire genome, with no gaps or hidden DNA, may give scientists a better understanding of human health, disease and evolution.
The newly complete genome doesn’t have gaps like the previous human reference genome. But it still has limitations, Wang says. The old reference genome is a conglomerate of more than 60 people’s DNA (SN: 3/4/21). “Not a single individual, or single cell on this planet, has that genome.” That goes for the new, complete genome, too. “It’s a quote-unquote fake genome,” says Wang, who was not involved with the project.
The new genome doesn’t come from a person either. It’s the genome of a complete hydatidiform mole, a sort of tumor that arises when a sperm fertilizes an empty egg and the father’s chromosomes are duplicated. The researchers chose to decipher the complete genome from a cell line called CHM13 made from one of these unusual tumors.
That decision was made for a technical reason, says geneticist Karen Miga of the University of California, Santa Cruz. Usually, people get one set of chromosomes from their mother and another set from their father. So “we all have two genomes in every cell.”
If putting together a genome is like assembling a puzzle, “you essentially have two puzzles in the same box that look very similar to each other,” says Miga, borrowing an analogy from a colleague. Researchers would have to sort the two puzzles before piecing them together. “Genomes from hydatidiform moles don’t present that same challenge. It’s just one puzzle in the box.”
The researchers did have to add the Y chromosome from another person, because the sperm that created the hydatidiform mole carried an X chromosome.
Even putting one puzzle together is a Herculean task. But new technologies that allow researchers to put DNA bases — represented by the letters A, T, C and G — in order, can spit out stretches up to more than 100,000 bases long. Just as children’s puzzles are easier to solve because of larger and fewer pieces, these “long reads” made assembling the bits of the genome easier, especially in repetitive parts where just a few bases might distinguish one copy from another. The bigger pieces also allowed researchers to correct some mistakes in the old reference genome.
What did they find? For starters, the newly deciphered DNA contains the short arms of chromosomes 13, 14, 15, 21 and 22. These “acrocentric chromosomes” don’t resemble nice, neat X’s the way the rest of the chromosomes do. Instead, they have a set of long arms and one of nubby short arms.
The length of the short arms belies their importance. These arms are home to rDNA genes, which encode rRNAs, which are key components of complex molecular machines called ribosomes. Ribosomes read genetic instructions and build all the proteins needed to make cells and bodies work. There are hundreds of copies of these rDNA regions in every person’s genome, an average of 315, but some people have more and some fewer. They’re important for making sure cells have protein-building factories at the ready.
“We didn’t know what to expect in these regions,” Miga says. “We found that every acrocentric chromosome, and every rDNA on that acrocentric chromosome, had variants, changes to the repeat unit that was private to that particular chromosome.”
By using fluorescent tags, Eichler and colleagues discovered that repetitive DNA next to the rDNA regions — and perhaps the rDNA too — sometimes switches places to land on another chromosome, the team reports in Science. “It’s like musical chairs,” he says. Why and how that happens is still a mystery.
The complete genome also contains 3,604 genes, including 140 that encode proteins, that weren’t present in the old, incomplete genome. Many of those genes are slightly different copies of previously known genes, including some that have been implicated in brain evolution and development, autism, immune responses, cancer and cardiovascular disease. Having a map of where all these genes lie may lead to a better understanding of what they do, and perhaps even of what makes humans human.
One of the biggest finds may be the structure of all of the human centromeres. Centromeres, the pinched portions which give most chromosomes their characteristic X shape, are the assembly points for kinetochores, the cellular machinery that divvies up DNA during cell division. That’s one of the most important jobs in a cell. When it goes wrong, birth defects, cancer or death can result. Researchers had already deciphered the centromeres of fruit flies and the human 8, X and Y chromosomes (SN: 5/17/19), but this is the first time that researchers got a glimpse of the rest of the human centromeres.
The structures are mostly head-to-tail repeats of about 171 base pairs of DNA known as alpha satellites. But those repeats are nestled within other repeats, creating complex patterns that distinguish each chromosome’s individual centromere, Miga and colleagues describe in Science. Knowing the structures will help researchers learn more about how chromosomes are divvied up and what sometimes throws off the process. Researchers also now have a more complete map of epigenetic marks — chemical tags on DNA or associated proteins that may change how genes are regulated. One type of epigenetic mark, known as DNA methylation, is fairly abundant across the centromeres, except for one spot in each chromosome called the centromeric dip region, Winston Timp, a biomedical engineer at Johns Hopkins University and colleagues report in Science.
Those dips are where kinetochores grab the DNA, the researchers discovered. But it’s not yet clear whether the dip in methylation causes the cellular machinery to assemble in that spot or if assembly of the machinery leads to lower levels of methylation.
Examining DNA methylation patterns in multiple people’s DNA and comparing them with the new reference revealed that the dips occur at different spots in each person’s centromeres, though the consequences of that aren’t known.
About half of genes implicated in the evolution of humans’ large, wrinkly brains are found in multiple copies in the newly uncovered repetitive parts of the genome (SN: 2/26/15). Overlaying the epigenetic maps on the reference allowed researchers to figure out which of many copies of those genes were turned on and off, says Ariel Gershman, a geneticist at Johns Hopkins University School of Medicine.
“That gives us a little bit more insight into which of them are actually important and playing a functional role in the development of the human brain,” Gershman says. “That was exciting for us, because there’s never been a reference that was accurate enough in these [repetitive] regions to tell which gene was which, and which ones are turned on or off.”
What is next? One criticism of genetics research is that it has relied too heavily on DNA from people of European descent. CHM13 also has European heritage. But researchers have used the new reference to discover new patterns of genetic diversity. Using DNA data collected from thousands of people of diverse backgrounds who participated in earlier research projects compared with the T2T reference, researchers more easily and accurately found places where people differ, McCoy and colleagues report in Science.
The Telomere-to-Telomere Consortium has now teamed up with Wang and his colleagues to make complete genomes of 350 people from diverse backgrounds (SN: 2/22/21). That effort, known as the pangenome project, is poised to reveal some of its first findings later this year, Wang says.
McCoy and Timp say that it may take some time, but eventually, researchers may switch from using the old reference genome to the more complete and accurate T2T reference. “It’s like upgrading to a new version of software,” Timp says. “Not everyone is going to want to do it right away.”
The completed human genome will also be useful for researchers studying other organisms, says Amanda Larracuente, an evolutionary geneticist at the University of Rochester in New York who was not involved in the project. “What I’m excited about is the techniques and tools this team has developed, and being able to apply those to study other species.”
Eichler and others already have plans to make complete genomes of chimpanzees, bonobos and other great apes to learn more about how humans evolved differently than apes did. “No one should see this as the end,” Eichler says, “but a transformation, not only for genomic research but for clinical medicine, though that will take years to achieve.”
Leeches suck. Most people try to avoid them. But in the summer of 2016, park rangers in China’s Ailaoshan Nature Reserve went hunting for the little blood gluttons.
For months, the rangers searched through the reserve’s evergreen forest, gathering tens of thousands of leeches by hand and sometimes plucking the slimy parasites from the rangers’ own skin. Each time the rangers found a leech, they would place it into a little, preservative-filled tube, tuck the tube into a hip pack and carry on. The work could help aid conservation efforts, at Ailaoshan and elsewhere.
There are many ways to measure how much effort goes into wildlife conservation, but it’s difficult to assess the success of that effort, even in protected areas, says Douglas Yu, an ecologist at the Kunming Institute of Zoology in China. But bloodthirsty worms may be just the tool for the job. Leeches aren’t picky eaters — they’ll feast on the blood of many different creatures, from amphibians to mammals to fish. Scientists have shown they can extract animal DNA from blood that leeches and other bloodsucking creatures have ingested, what’s known as invertebrate-derived DNA, or iDNA, and identify the source animal.
And some researchers had suggested that iDNA, a type of environmental DNA, could be used to trace the ranges of animals in an area, Yu says (SN: 1/18/22). “We thought we would just actually just try to do it.”
Enlisting 163 park rangers, Yu and colleagues deputized the leech-hunters with gathering the parasites along rangers’ regular patrol routes, which covered all 172 areas of the reserve.
Three months later, the rangers had gathered 30,468 leeches. After extracting and analyzing animal DNA from the leeches’ blood meals, Yu and colleagues detected the presence of 86 different species, including Asiatic black bears, domestic cattle, endangered Yunnan spiny frogs and, of course, humans.
What’s more, the iDNA gave clues to where the animals preferred to roam, the researchers report March 23 in Nature Communications. Wildlife biodiversity was greatest in the reserve’s high-altitude interior, the researchers found, while domestic cattle, sheep and goats were more abundant in the reserve’s lower, more accessible zones. Because most of the wild species detected should be able to inhabit all parts of the reserve, the dichotomy suggests that human activity may be pushing wildlife away from certain areas, Yu says.
Compared with other methods for surveying wildlife, using iDNA from leeches is “really cost- and time-efficient and doesn’t require a lot of expertise,” says Arthur Kocher, an ecologist at the Max Planck Institute for the Science of Human History in Jena, Germany, who was not involved in the study.
Camera traps, for instance, are triggered only by animals of large enough size, and the instruments are expensive. Sight-based surveys require trained observers. With leeches, Kocher says, “there are clear advantages.”
Yu and Kocher both suspect that leeches and other bloodsucking critters, such as carrion flies or mosquitoes, will become more popular wildlife surveillance tools in the future. People are becoming more aware of what iDNA brings to the table, Yu says.
Recurring climate changes may have orchestrated where Homo species lived over the last 2 million years and how humankind evolved.
Ups and downs in temperature, rainfall and plant growth promoted ancient hominid migrations within and out of Africa that fostered an ability to survive in unfamiliar environments, say climate physicist and oceanographer Axel Timmermann and colleagues. Based on how the timing of ancient climate variations matched up with the comings and goings of different fossil Homo species, the researchers generated a novel — and controversial — outline of human evolution. Timmermann, of Pusan National University in Busan, South Korea, and his team present that scenario April 13 in Nature.
Here’s how these scientists tell the story of humankind, starting roughly 2 million years ago. By that time, Homo erectus had already begun to roam outside Africa, while an East African species called H. ergaster stuck close to its home region. H. ergaster probably evolved into a disputed East African species called H. heidelbergensis, which split into southern and northern branches between 850,000 and 600,000 years ago. These migrations coincided with warmer, survival-enhancing climate shifts that occur every 20,000 to 100,000 years due to variations in Earth’s orbit and tilt that modify how much sunlight reaches the planet. Then, after traveling north to Eurasia, H. heidelbergensis possibly gave rise to Denisovans around 430,000 years ago, the researchers say. And in central Europe, harsh habitats created by recurring ice ages spurred the evolution of H. heidelbergensis into Neandertals between 400,000 and 300,000 years ago. Finally, in southern Africa between 310,000 and 200,000 years ago, increasingly harsh environmental conditions accompanied a transition from H. heidelbergensis to H. sapiens, who later moved out of Africa.
But some researchers contend that H. heidelbergensis, as defined by its advocates, contains too many hard-to-categorize fossils to qualify as a species.
An alternative view to the newly proposed scenario suggests that, during the time that H. heidelbergensis allegedly lived, closely related Homo populations periodically split up, reorganized and bred with outsiders, without necessarily operating as distinct biological species (SN: 12/13/21). In this view, mating among H. sapiens groups across Africa starting as early as 500,000 years ago eventually produced a physical makeup typical of people today. If so, that would undermine the validity of a neatly branching evolutionary tree of Homo species leading up to H. sapiens, as proposed by Timmermann’s group.
The new scenario derives from a computer simulation of the probable climate over the last 2 million years, in 1,000-year intervals, across Africa, Asia and Europe. The researchers then examined the relationship between simulated predictions of what ancient habitats were like in those regions and the dates of known hominid fossil and archaeological sites. Those sites range in age from around 2 million to 30,000 years old.
Previous fossil evidence indicates that H. erectus spread as far as East Asia and Java (SN: 12/18/19). Timmermann’s climate simulations suggest that H. erectus, as well as H. heidelbergensis and H. sapiens, adapted to increasingly diverse habitats during extended travels. Those migrations stimulated brain growth and cultural innovations that “may have made [all three species] the global wanderers that they were,” Timmermann says.
The new habitat simulations also indicate that H. sapiens was particularly good at adjusting to hot, dry regions, such as northeastern Africa and the Arabian Peninsula.
Climate, habitat and fossil data weren’t sufficient to include additional proposed Homo species in the new evolutionary model, including H. floresiensis in Indonesia (SN: 3/30/16) and H. naledi in South Africa (SN: 5/9/17).
It has proven difficult to show more definitively that ancient environmental changes caused transitions in hominid evolution. For instance, a previous proposal that abrupt climate shifts resulted in rainy, resource-rich stretches of southern Africa’s coast, creating conditions where H. sapiens then evolved (SN: 3/31/21), still lacks sufficient climate, fossil and other archaeological evidence.
Paleoanthropologist Rick Potts of the Smithsonian Institution in Washington, D.C., has developed another influential theory about how climate fluctuations influenced human evolution that’s still open to debate. A series of climate-driven booms and busts in resource availability, starting around 400,000 years ago in East Africa, resulted in H. sapiens evolving as a species with a keen ability to survive in unpredictably shifting environments, Potts argues (SN: 10/21/20). But the new model indicates that ancient H. sapiens often migrated into novel but relatively stable environments, Timmermann says, undermining support for Potts’ hypothesis, known as variability selection.
The new findings need to be compared with long-term environmental records at several well-studied fossil sites in Africa and East Asia before rendering a verdict on variability selection, Potts says.
The new model “provides a great framework” to evaluate ideas such as variability selection, says paleoclimatologist Rachel Lupien of Lamont-Doherty Earth Observatory in Palisades, N.Y. That’s especially true, Lupien says, if researchers can specify whether climate and ecosystem changes that played out over tens or hundreds of years were closely linked to ancient Homo migrations.
For now, much remains obscured on the ancient landscape of human evolution.
Punishing headbutts damage the brains of musk oxen. That observation, made for the first time and reported May 17 in Acta Neuropathologica, suggests that a life full of bell-ringing clashes is not without consequences, even in animals built to bash.
Although a musk ox looks like a dirty dust mop on four tiny hooves, it’s formidable. When charging, it can reach speeds up to 60 kilometers an hour before ramming its head directly into an oncoming head. People expected that musk oxen brains could withstand these merciless forces largely unscathed, “that they were magically perfect,” says Nicole Ackermans of the Icahn School of Medicine at Mount Sinai in New York City. “No one actually checked.” In fact, the brains of three wild musk oxen (two females and one male) showed signs of extensive damage, Ackermans and her colleagues found. The damage was similar to what’s seen in people with chronic traumatic encephalopathy, a disorder known to be caused by repetitive head hits (SN: 12/13/17). In the musk ox brains, a form of a protein called tau had accumulated in patterns that suggested brain bashing was to blame.
In an unexpected twist, the brains of the females, who hit heads less frequently than males, were worse off than the male’s. The male body, with its heavier skull, stronger neck muscles and forehead fat pads, may cushion the blows to the brain, the researchers suspect.
The results may highlight an evolutionary balancing act; the animals can endure just enough brain damage to allow them to survive and procreate. High-level brainwork may not matter much, Ackermans says. “Their day-to-day life is not super complicated.”
Six years ago, tour guide Brenden Miles was traveling down the Kinabatangan River in the Malaysian part of Borneo, when he spotted an odd-looking primate he had never seen before. He snapped a few pictures of the strange monkey and, on reaching home, checked his images.
“At first, I thought it could be a morph of the silvered leaf monkey,” meaning a member of the species with rare color variation, Miles says. But then he noticed other little details. “Its nose was long like that of a proboscis monkey, and its tail was thicker than that of a silvered leaf [monkey],” he says. He posted a picture of the animal on Facebook and forgot all about it.
Now, an analysis of that photo and others suggests that the “mystery monkey” is a hybrid of two distantly related primate species that share the same fragmented habitat. The putative offspring was produced when a male proboscis monkey (Nasalis larvatus) mated with a female silvered leaf monkey (Trachypithecus cristatus), researchers suggest April 26 in the International Journal of Primatology. And that conclusion has the scientists worried about the creature’s parent species.
Hybridization between closely related organisms has been observed in captivity and occasionally in the wild (SN: 7/23/21). “But hybridization across genera, that’s very rare,” says conservation practitioner Ramesh Boonratana, the regional vice-chair for Southeast Asia for the International Union for Conservation of Nature’s primate specialist group.
Severe habitat loss, fragmentation and degradation caused by expanding palm oil plantations along the Kinabatangan River could explain how the possible hybrid came to be, says primatologist Nadine Ruppert.
“Different species — even from the same genus — when they share a habitat, they may interact with each other, but they may usually not mate. This kind of cross-genera hybridization happens only when there is some ecological pressure,” says Ruppert, of the Universiti Sains Malaysia in Penang Island.
The state of Sabah, where Kinabatangan River is located, lost about 40 percent of its forest cover from 1973 to 2010, with logging and palm oil plantations being the main drivers of deforestation, a study in 2014 found. “In certain areas, both [monkey] species are confined to small forest fragments along the river,” Ruppert says. This leads to competition for food, mates and other resources. “The animals cannot disperse and, in this case, the male of the larger species — the proboscis monkey — can easily displace the male silvered leaf monkey.”
Since 2016, there have been some more documented sightings of the mystery monkey, though these have been sporadic. The infrequent sightings and the COVID-19 pandemic has, for now, prevented researchers from gathering fecal samples for genetic analysis to reveal the monkey’s identity. Instead, Ruppert and colleagues compared images of the possible hybrid with those of the parent species, both visually as well as by using limb ratios. “If the individual was from one of the two parent species, all its measurements would be similar to that of one species,” Ruppert says. “But that is not the case with this animal.”
A photograph of a male proboscis monkey mating with a female silvered leaf monkey, along with anecdotes from boat operators and tour guides about a single male proboscis monkey hanging around a troop of female silvered leaf monkeys, has added further weight to the researchers’ conclusion.
The mystery monkey is generating a lot of excitement in the area, but Ruppert is concerned for the welfare of both proposed parent species. The International Union for Conservation of Nature classifies proboscis monkeys as endangered and silvered leaf monkeys as vulnerable. “The hybrid is gorgeous, but we don’t want to see more of them,” Ruppert says. “Both species should have a large enough habitat, dispersal opportunities and enough food to conduct their natural behaviors in the long term.”
Increasing habitat loss or fragmentation in Borneo and elsewhere as a result of changing land uses or climate change could lead to more instances of mating — or at least, attempts at mating — between species or even genera, Boonratana says.
The mystery monkey was last photographed in September of 2020 with swollen breasts and holding a baby, suggesting that the animal is a fertile female. That’s another surprising development, the researchers say, because most hybrids tend to be sterile.
One million deaths. That is now roughly the toll of COVID-19 in the United States. And that official milestone is almost certainly an undercount. The World Health Organization’s data suggest that this country hit a million deaths early in the year.
Whatever the precise dates and numbers, the crisis is enormous. The disease has taken the lives of more than 6 million people worldwide. Yet our minds cannot grasp such large numbers. Instead, as we go further out on a mental number line, our intuitive understanding of quantities, or number sense, gets fuzzier. Numbers simply start to feel big. Consequently, people’s emotions do not grow stronger as crises escalate. “The more who die, the less we care,” psychologists Paul Slovic and Daniel Västfjäll wrote in 2014. But even as our brains struggle to grasp big numbers, the modern world is awash in such figures. Demographic information, funding for infrastructure and schools, taxes and national deficits are all calculated in the millions, billions and even trillions. So, too, are the human and financial losses from global crises, including the pandemic, war, famines and climate change. We clearly have a need to conceptualize big numbers. Unfortunately, the slow drumbeat of evolution means our brains have yet to catch up with the times.
Our brains think 5 or 6 is big. Numbers start to feel big surprisingly fast, says educational neuroscientist Lindsey Hasak of Stanford University. “The brain seems to consider anything larger than five a large number.”
Other scientists peg that value at four. Regardless of the precise pivot from small to big, researchers agree that humans, along with fish, birds, nonhuman primates and other species, do remarkably well at identifying really, really small quantities. That’s because there’s no counting involved. Instead, we and other species quickly recognize these minute quantities through a process called “subitizing” — that is, we look and we immediately see how many.
“You see one apple, you see three apples, you would never mistake that. Many species can do this,” says cognitive scientist Rafael Núñez of the University of California, San Diego.
When the numbers exceed subitizing range — about four or five for humans in most cultures — species across the biological spectrum can still compare approximate quantities, says cognitive scientist Tyler Marghetis of the University of California, Merced.
Imagine a hungry fish eyeing two clumps of similarly sized algae. Because both of those options will make “awesome feasts,” Marghetis says, the fish doesn’t need to waste limited cognitive resources to differentiate between them. But now imagine that one clump contains 900 leaves and the other 1,200 leaves. “It would make evolutionary sense for the fish to try to make that approximate comparison,” Marghetis says. Scientists call this fuzzy quantification ability an “approximate number sense.” Having the wherewithal to estimate and compare quantities gives animals a survival edge beyond just finding food, researchers wrote in a 2021 review in the Journal of Experimental Biology. For example, when fish find themselves in unfamiliar environments, they consistently join the larger of two schools of fish.
The approximate number system falls short, however, when the quantities being compared are relatively similar, relatively large or both. Comparing two piles, one with five coins and the other with nine coins, is easy. But scale those piles up to 900,005 coins and 900,009 coins, and the task becomes impossible. The same goes for when the U.S. death toll from COVID-19 goes from 999,995 to 999,999.
We can improve our number sense — to a point. The bridge between fuzzy approximation and precision math appears to be language, Núñez says.
Because the ability to approximate numbers is universal, every known language has words and phrases to describe inexact quantities, such as a lot, a little and a gazillion. “For example, if a boy is said to have a ‘few’ oranges and a girl ‘many’ oranges, a safe inference — without the need of exact calculations — is that the girl has more oranges than the boy,” Núñez writes in the June 1, 2017 Trends in Cognitive Science.
And most cultures have symbols or words for values in the subitizing range, but not necessarily beyond that point, Núñez says. For instance, across 193 languages in hunting and gathering communities, just 8 percent of Australian languages and 39 percent of African languages have symbols or words beyond five, researchers reported in the 2012 Linguistic Typology. The origin of counting beyond subitizing range, and the complex math that follows, such as algebra and calculus, remains unclear. Núñez and others suspect that cultural practices and preoccupations, such as keeping track of agricultural products and raw materials for trade, gave rise to more complex numerical abilities. As math abilities developed, people became adept at conceptualizing numbers up to 1,000 due to lived experience, says cognitive scientist David Landy. Those experiences could include getting older, traveling long distances or counting large quantities of money.
Regular experiences, however, rarely hit the really big number range, says Landy, a senior data scientist at Netflix in San Francisco. Most people, he says, “get no experience like that for a million.”
Numbers that exceed our experience perplex us. When big numbers exceed our lived experiences, or move into the abstract, our minds struggle to cope. For instance, with number sense and language so deeply intertwined, those seemingly benign commas in big numbers and linguistic transitions from thousands to millions or millions to billions, can trip us up in surprising ways.
When Landy and his team ask participants, often undergraduates or adults recruited online, to place numbers along a number line, they find that people are very accurate at placing numbers between 1 and 1,000. They also perform well from 1 million to 900 million. But when they change the number line endpoints to, say, 1,000 and 1 billion, people struggle at the 1 million point, Landy and colleagues reported in the March 2017 Cognitive Science.
“Half the people are putting 1 million closer to 500 million than 1,000,” Landy says. “They just don’t know how big a million is.” Landy believes that as people transition from their lived experiences in the thousands to the more abstract world of 1 million, they reset their mental number lines. In other words, 1 million feels akin to one, 2 million to two and so on.
Changing our notations might prevent that reset, Landy says. “You might be better off writing ‘a thousand thousand’ than ‘1 million’ because that’s easier to compare to 900,000.” The British used to do this with what people in the U.S. now call a trillion, which they called a million million.
Without comprehension, extreme numbers foster apathy. Our inability to grasp big numbers means that stories featuring a single victim, often a child, are more likely to grab our attention than a massive crisis — a phenomenon known as the identifiable victim effect.
For instance, on September 2, 2015, Aylan Kurdi, a 2-year-old refugee of the Syrian Civil War, was on a boat with his family crossing the Mediterranean Sea. Conservative estimates at the time put the war’s death toll at around 250,000 people. Kurdi’s family was trying to escape, but when their overcrowded boat capsized, the boy drowned, along with his brother and mother. The next day a picture of the infant lying dead on a Turkish beach hit the front pages of newspapers around the world.
No death up until that point had elicited public outcry. That photograph of a single innocent victim, however, proved a catalyst for action. Charitable contributions to the Swedish Red Cross, which had created a fund for Syrian refugees in August 2015, skyrocketed. In the week leading up to the photo’s appearance, daily donations averaged 30,000 Swedish krona, or roughly $3,000 today; in the week after the photo appeared, daily donations averaged 2 million Swedish krona, or roughly $198,500. Paul Slovic, of the University of Oregon, Eugene, Daniel Västfjäll, of Linköping University, Sweden, and colleagues reported those results in 2017 in Proceedings of the National Academy of Sciences. Earlier research shows that charitable giving, essentially a proxy for compassion, decreases even when the number of victims goes from one to two. The flip side, however, is that psychologists and others can use humans’ tendency to latch onto iconic victims to reframe large tragedies, says Deborah Small, a psychologist at the University of Pennsylvania.
Some research suggests that this power of one need not focus on a single individual. For instance, when people were asked to make hypothetical donations to save 200,000 birds or a flock of 200,000 birds, people gave more money to the flock than the individual birds, researchers reported in the 2011 E-European Advances in Consumer Research.
Framing the current tragedy in terms of a single unit likewise makes sense, Västfjäll says. Many people react differently, he says, to hearing ‘1 million U.S. citizens dead of COVID’ vs. ‘1 million people, roughly the equivalent of the entire city of San José, Calif., have died from COVID.’
Milestones do still matter, even if we can’t feel them. Kurdi’s photo sparked an outpouring of empathy. But six weeks after it was published, donations had dropped to prephoto levels — what Västfjäll calls “the half-life of empathy.”
That fade to apathy over time exemplifies a phenomenon known as hedonic adaptation, or humans’ ability to eventually adjust to any situation, no matter how dire. We see this adaptation with the pandemic, Small says. A virus that seemed terrifying in March 2020 now exists in the background. In the United States, masks have come off and people are again going out to dinner and attending large social events (SN: 5/17/22).
One of the things that can penetrate this apathy, however, is humans’ tendency to latch onto milestones — like 1 million dead from COVID-19, Landy says. “We have lots of experience with small quantities carrying emotional impact. They are meaningful in our lives. But in order to think about big numbers, we have to go to a more milestone frame of mind.” That’s because our minds have not caught up to this moment in time where big numbers are everywhere.
And even if we cannot feel that 1 million milestone, or mourn the more than 6 million dead worldwide, the fact that we even have the language for numbers beyond just 4 or 5 is a feat of human imagination, Marghetis says. “Maybe we are not having an emotional response to [that number], but at least we can call it out. That’s an amazing power that language gives us.”
A speck of gold dancing to a pipe organ’s tune has helped solve a long-standing mystery: why certain wind instruments violate a mathematical formula that should describe their sound.
In 1860, physicist Hermann von Helmholtz — famous for his law of the conservation of energy — devised an equation relating the wavelength of a pipe’s fundamental tone (the lowest frequency at which it resonates) to pipe length (SN: 3/31/28). Generally, the longer a pipe is, the lower its fundamental tone will be.
But the equation doesn’t work in practice. A pipe’s fundamental tone always sounds lower than the pipe’s length suggests it should according to Helmholtz’s formula. Fixing this problem requires adding an “end correction” to the equation. In the case of open-ended pipes such as flutes and those of organs, the end correction is 0.6 times the radius of the pipe. Why this was, nobody could figure out.
A break in the case came in 2010. Instrument builder and restorer Bernhardt Edskes of Wohlen, Switzerland was tuning an organ when he spotted a piece of gold that had come loose from a pipe’s gilded lip. Air pumping through the pipe should have carried away the gold. Instead, it seemed to be trapped in a vortex just above the pipe’s upper rim.
Edskes told his friend, physicist Leo van Hemmen of the Technical University of Munich, about the observation. Together with colleagues from Munich and Wageningen University in the Netherlands, they studied how air moves through playing organ pipes using cigarette smoke.
When an organ pipe sounds, a vortex indeed forms over the pipe’s rim, the team reported March 14 in Chicago at a meeting of the American Physical Society. What’s more, this vortex is capped by a hemisphere of resonating air. This vibrating air cap, van Hemmen says, is the long-sought explanation for the “end correction.” The cap effectively lengthens the organ pipe by the exact amount that must be tacked on to Helmholtz’s formula to explain the pipe’s fundamental tone.
