Mammals’ bodies outpaced their brains right after the dinosaurs died

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.

We finally have a fully complete human genome

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.”

A UN report says stopping climate change is possible but action is needed now

It doesn’t have to be this way.

The world already has the know-how and tools to dramatically reduce emissions from fossil fuels — but we need to use those tools immediately if we hope to forestall the worst impacts of climate change. That’s the message of the third and final installment of the massive sixth assessment of climate science by the United Nations’ Intergovernmental Panel on Climate Change, which was released April 4.

“We know what to do, we know how to do it, and now it’s up to us to take action,” said sustainable energy researcher Jim Skea of Imperial College London, who cochaired the report, at a news event announcing its release.
Earth is on track to warm by an average of about 3.2 degrees Celsius above preindustrial levels by the end of the century (SN: 11/26/19). Altering that course and limiting warming to 1.5 degrees or even 2 degrees means that global fossil fuel emissions will need to peak no later than the year 2025, the new report states.

Right now, meeting that goal looks extremely unlikely. National pledges to reduce fossil fuel emissions to date amount to “a litany of broken climate promises,” said United Nations Secretary-General António Guterres at the event.

The previous two installments of the IPCC’s sixth assessment described how climate change is already fueling extreme weather events around the globe — and noted that adaptation alone will not be enough to shield people from those hazards (SN: 8/9/21; SN: 2/28/22).

The looming climate crisis “is horrifying, and I don’t want to sugarcoat that,” says Bronson Griscom, a forest ecologist and the director of Natural Climate Solutions at the environmental organization Conservation International, based in Arlington, Va.

But Griscom, who was not an author on the new IPCC report, says its findings also give him hope. It’s “what I would call a double-or-nothing bet that we’re confronted with right now,” he says. “There [are] multiple ways that this report is basically saying, ‘Look, if we don’t do anything, it’s increasingly grim.’ But the reasons to do something are incredibly powerful and the tools in the toolbox are very powerful.”

Tools in the toolbox
Those tools are strategies that governments, industries and individuals can use to cut emissions immediately in multiple sectors of the global economy, including transportation, energy, building, agriculture and forestry, and urban development. Taking immediate advantage of opportunities to reduce emissions in each of those sectors would halve global emissions by 2030, the report states.

Consider the transportation sector, which contributed 15 percent of human-related greenhouse gas emissions in 2019. Globally, electric vehicle sales have surged in the last few years, driven largely by government policies and tougher emissions laws for the auto industry (SN: 12/22/21).

If that surge continues, “electric vehicles offer us the greatest potential [to reduce transportation emissions on land], as long as they’re combined with low or zero carbon electricity sources,” Diana Ürge-Vorsatz, the vice chair of the IPCC’s Working Group III, said at the news event. But for aviation and long-haul shipping, which are more difficult to electrify, reduced carbon emissions could be achieved with low-carbon hydrogen fuels or biofuels, though these alternatives require further research and development.

Then there are urban areas, which are contributing a growing proportion of global greenhouse gas emissions, from 62 percent in 2015 to between 67 and 72 percent in 2020, the report notes. In established cities, buildings can be retrofitted, renovated or repurposed to make city layouts more walkable and provide more accessible public transportation options.

And growing cities can incorporate energy-efficient infrastructure and construct buildings using zero-emissions materials. Additionally, urban planners can take advantage of green roofs, urban forests, rivers and lakes to help capture and store carbon, as well as provide other climate benefits such as cleaner air and local cooling to counter urban heat waves (SN: 4/3/18).

Meanwhile, “reducing emissions in industry will involve using materials and energy more efficiently, reusing and recycling products and minimizing waste,” Ürge-Vorsatz said.

As for agriculture and forestry, these and other land-use industries contribute about 22 percent of the world’s greenhouse gas emissions, with half of those emissions coming from deforestation (SN: 7/13/21). So reforestation and reduced deforestation are key to flipping the balance between CO₂ emissions and removal from the atmosphere (SN: 7/9/21; SN: 1/3/22). But there are a lot of other strategies that the world can employ at the same time, the report emphasizes. Better management of forests, coastal wetlands, grasslands and other ecosystems, more sustainable crop and livestock management, soil carbon management in agriculture and agroforestry can all bring down emissions (SN: 7/14/21).

The report also includes, for the first time in the IPCC’s reports, a chapter on the “untapped potential” of lifestyle changes to reduce emissions. Such changes include opting for walking or cycling or using public transportation rather than driving, shifting toward plant-based diets and reducing air travel (SN: 5/14/20).

Those lifestyle changes could reduce emissions by 40 to 70 percent by 2050, the report suggests. To enable those changes, however, government policies, infrastructure and technology would need to be in place.

Government policies are also key to financing these transformational changes. Globally, the investment in climate-related technologies needs to ramp up, and quickly, to limit warming below 2 degrees C, the report states. Right now, investments are three to six times lower than they need to be by 2030. And a combination of public and private investments will be essential to aiding the transition away from fossil fuels and toward renewable energy in developing nations (SN: 1/25/21).

Future strategies
Still, reducing emissions alone won’t be enough: We will need to actively remove carbon from the atmosphere to achieve net zero emissions and keep the planet well below 2 degrees C of warming, the report notes. “One thing that’s clear in this report, as opposed to previous reports, is that carbon removal is going to be necessary in the near term,” says Simon Nicholson, director of the Institute for Carbon Removal Law and Policy at American University in Washington, D.C., who was not involved in the report.

Such strategies include existing approaches such as protecting or restoring carbon dioxide–absorbing forests, but also technologies that are not yet widely available commercially, such as directly capturing carbon dioxide from the air, or converting the gas to a mineral form and storing it underground (SN: 12/17/18).

These options are still in their infancy, and we don’t know how much of an impact they’ll have yet, Nicholson says. “We need massive investment now in research.”

An emphasis on acting “now,” on eliminating further delay, on the urgency of the moment has been a recurring theme through all three sections of the IPCC’s sixth assessment report released over the last year. What impact these scientists’ stark statements will have is unclear.

But “the jury has reached a verdict, and it is damning,” U.N. Secretary-General Guterres said. “If you care about justice and our children’s future, I am appealing directly to you.”

A star nicknamed ‘Earendel’ may be the most distant yet seen

A chance alignment may have revealed a star from the universe’s first billion years.

If confirmed, this star would be the most distant one ever seen, obliterating the previous record (SN: 7/11/17). Light from the star traveled for about 12.9 billion years on its journey toward Earth, about 4 billion years longer than the former record holder, researchers report in the March 30 Nature. Studying the object could help researchers learn more about the universe’s composition during that early, mysterious time.

“These are the sorts of things that you only hope you could discover,” says astronomer Katherine Whitaker of the University of Massachusetts Amherst, who was not part of the new study.
The researchers found the object while analyzing Hubble Space Telescope images of dozens of clusters of galaxies nearer to Earth. These clusters are so massive that they bend and focus the light from more distant background objects, what’s known as gravitational lensing (SN: 10/6/15).

In images of one cluster, astronomer Brian Welch of Johns Hopkins University and colleagues noticed a long, thin, red arc. The team realized that the arc was a background galaxy whose light the cluster had warped and amplified.

Atop that red arc is a bright spot that is too small to be a small galaxy or a star cluster, the researchers say. “We stumbled into finding that this was a lensed star,” Welch says.

The researchers estimate that the star’s light originates from only 900 million years after the Big Bang, which took place about 13.8 billion years ago.

Welch and his colleagues think that the object, which they poetically nicknamed “Earendel” from the old English word meaning “morning star” or “rising light,” is a behemoth with at least 50 times the mass of the sun. But the researchers can’t pin down that value, or learn more about the star or even confirm that it is a star, without more detailed observations.

The researchers plan to use the recently launched James Webb Space Telescope to examine Earendel (SN: 10/6/21). The telescope, also known as JWST, will begin studying the distant universe this summer.

JWST may uncover objects from even earlier times in the universe’s history than what Hubble can see because the new telescope will be sensitive to light from more distant objects. Welch hopes that the telescope will find many more of these gravitationally lensed stars. “I’m hoping that this record won’t last very long.”