Many cultures associate twins with health and vitality, while others see them as a philosophical reminder of the duality of life and death, good and evil. 
| Photo Credit: Tesla Monson/The Conversation

Twins have been rare in human history and for that reason can seem special. Many cultures associate twins with health and vitality, while others see them as a philosophical reminder of the duality of life and death, good and evil. Some famous twins are credited with the birth of nations, others are described as deities.

Our recent research suggests that twins were actually the norm much further back in primate evolution, rather than an unusual occurrence worthy of note. Despite the fact that almost all primates today, including people, usually give birth to just one baby, our most recent common ancestor, which roamed North America about 60 million years ago, likely gave birth to twins as the standard.

We have been researching the evolution of primate litter size – how many babies grow during each pregnancy – for the past several years. To study mammal evolution and reproductive life history, we use skeletal collections, both fossil and recently living.

In addition to being an anthropologist, one of us (Tesla) is the mother of twin girls. That’s led to a personal and not just scientific interest in this topic: When did twin pregnancies become uncommon?

Reconstructing litter size in the past

The best way we have to reconstruct the history of litter size is to map the known litter size of as many species as possible across the mammalian family tree and then use mathematical algorithms to look for patterns. But outside of rare events where entire animal families are fossilized together, it is extraordinarily difficult to assess litter size for extinct species from the skeleton alone. So we instead collect data on as many living mammals as possible.

We searched a wide variety of public databases, including AnAge: The Animal Ageing and Longevity Database, for information about how many offspring are commonly born to each species of mammal. We also noted additional data, including what the species’ average body size is at birth and at adulthood, as well as pregnancy duration.

After gathering all these data points for almost a thousand mammal species, we performed a series of statistical tests to quantify relationships between different traits. Our goal was to estimate the likely litter size of different mammalian ancestors: What were the odds of a singleton birth for each species at any given point in time?

The number of offspring a species has in a litter is phylogenetically conserved, meaning more similar in more closely related species. Deer tend to have one or two offspring, while canids and felids tend to have many more babies in each litter.

Almost all primate species give birth to just one baby, although there are exceptions. Several of the wet-nosed primates – including lemurs, lorises and galagos – and almost all of the marmosets and tamarins from South America give birth to twins.

Prior to our work, researchers thought these distinctive twin-bearing primates must be what evolutionary biologists call derived, or different, from the more common, ancestral trait. But our research flips that narrative on its head: It’s actually the singleton-bearing primates that are derived and distinctive. Further back in evolution, two babies at once was the norm. Our ancient primate ancestors gave birth to twins.

So, when did this evolutionary change in primate litter size occur?

The switch to singletons

Modern humans overwhelmingly birth just a single child – a rather large child with an even larger head. Human brain and body size is certainly connected to our ability to create and refine technologies. Paleoanthropologists have long been investigating what they call encephalization: an increase in brain size relative to body size over evolutionary time.

For primates, and especially humans, childhood learning is crucial. We propose that the switch from twins to singletons was critical for the evolution of large human babies with large brains that were capable of complex learning as infants and young children.

Based on mathematical modeling, the switch to singletons occurred early on, at least 50 million years ago. From there, many primate lineages, including ours, evolved to have increasingly larger bodies and brains.

Our new research also shows that the switch from birthing twins to birthing singletons happened multiple times in the primate lineage – a telltale signal that it was advantageous for primates to develop only one fetus per pregnancy. Because multifetal gestation requires more energy from the mother, and because the babies are born smaller, and often earlier, early primate ancestors who gave birth to just one large offspring may have been at a survival advantage.

Our findings don’t mean that having twins today is a disadvantage – although, as a mother of multiples, Tesla can certainly say it’s not easy. But having twins today is quite a different experience from our tiny primate ancestors birthing in the trees 60 million years ago.

Twinning today

Rates of twins have almost doubled in the U.S. over the past 50 years, due in part to advances in assistive reproductive technologies. Today, about 3% of live births are twins, although recent trends suggest a downturn in rates. The fact that women in the U.S. are routinely having kids in their 30s compounds this even further, since women in the later stages of fertility – that’s anyone over the age of 35 – are more likely to have twins.

But having twins can be dangerous for both the mother and babies. More than half of all twins in the U.S. are born prematurely. Many of them spend time in the neonatal intensive care unit.

Despite these risks, our research shows that twins are a critical part of our genetic history.

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

Dying coral reefs, rainforests transforming into savannas, grasslands turning into deserts – these are ecosystem “tipping points”, boundary lines we’re desperate not to cross.

In dynamic systems filled with life, these critical thresholds aren’t set in stone. Since organisms can evolve, the tipping points within these ecosystems might evolve too.

Most of us think of evolution as a glacial process, too slow to witness in a single lifetime. But evolution, especially in the microbial world, can happen very quickly. Consider antibiotic-resistant bacteria that emerge within years, or the COVID-causing virus evolving new variants in mere months. When the conditions are just right, evolution can go into overdrive — although that’s usually not good for us.

Our latest research, published today in Nature Ecology and Evolution, reveals the first experimental evidence that tipping point behaviour can indeed evolve, and evolve quickly. This raises an exciting prospect: could understanding the evolution of tipping points help us steer ecosystems away from collapse?

Equilibria, diversity and exclusion

Tipping points are critical thresholds where a small change in environmental conditions can lead to a dramatic and often irreversible shift in an ecosystem’s state. But what exactly does this mean?

An ecological community is a network of interacting species – plants, animals and microorganisms – that live in the same area and are interconnected through various relationships like predation, competition and symbiosis.

A healthy ecological community has a balanced mix of species that perform essential roles. They contribute to services like pollination, nutrient cycling, water purification and climate regulation.

When an ecosystem is pushed toward a tipping point due to stressors like climate change, habitat destruction or pollution, this balance is disrupted.

Crossing these thresholds often leads to less diverse communities that fail to perform essential functions. This results in species extinctions and a loss of vital ecosystem services.

Take coral reefs, for example. Corals have a symbiotic relationship with microscopic algae called zooxanthellae. Rising ocean temperatures push the ecosystem toward a tipping point: a critical thermal threshold beyond which corals expel the algae in a process known as coral bleaching.

If the stressful conditions persist, the ecosystem shifts from a vibrant, biodiverse reef to a barren underwater landscape. This can lead to the collapse of the entire reef ecosystem and the loss of marine life that relies on it.

So, we know tipping points are crucial for us to understand. But observing and mapping tipping point behaviour in these complex systems is extraordinarily difficult.

To explore how evolution might influence tipping points, we turned to the microscopic world.

Evolution in real time

Bacteria reproduce at astonishing speeds, allowing us to witness evolution as it happens. Over just 400 days, we guided a model microbial community of E. coli through 4,000 generations. This would be equivalent to about 100,000 years for humans.

One of the unique perks of working with microbes is our ability to freeze them, like biological snapshots in time. This means we can store the original “ancestral” community, and later revive it to compare with its evolved descendants.

By exposing both the ancestral and evolved communities to a range of environmental stresses, we could observe when they remained stable and when they collapsed.

We discovered that evolution can cut both ways.

As species co-evolved to become better adapted to benign conditions, they became more sensitive to stress. This had no impact until we exposed the community to adverse environmental conditions. This triggered unexpected destabilisation, bringing the tipping point closer, and causing collapse sooner than anticipated.

On the flip side, when we specifically evolved the community to withstand environmental stress, the species adapted in ways that allowed them to coexist under much harsher conditions. This effectively delayed the tipping point.

A new model

To illustrate these ecological changes, we developed a mathematical model that can allow for tipping points.

By tweaking key factors like competition levels, stress resistance and species growth rates, we could simulate how different evolutionary changes affect an entire ecosystem’s trajectory.

If we want to forecast the stability of ecosystems, having a way to include evolution is a significant step.

On the one hand, our results indicate that “directed evolution” – with humans helping species adapt to environmental changes – could bolster ecosystem resilience and prevent collapse.

On the other hand, our research warns us evolution isn’t always a saviour. For example, evolutionary changes that increase the dependence, or impact, of one species on another species may speed up destabilisation, making tipping points arrive sooner than we’d like.

As we grapple with unprecedented environmental challenges, understanding evolution’s role in ecosystem stability becomes more critical than ever. Just as evolution can help harmful bacteria outsmart our antibiotics, it can also shift the tipping points of entire ecosystems, for better or worse.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

In this undated image made from video provided by the BBC, hatchling marine iguanas. British naturalist Charles Darwin sketched out his theory of evolution in the 1859 book On the Origin of Species – proposing that biological species change over time through the acquisition of traits that favour survival and reproduction. Now 164 years later, nine scientists and philosophers on Monday proposed a new law of nature that includes the biological evolution described by Darwin as a vibrant example of a much broader phenomenon. Image for Representation.
| Photo Credit: AP

When British naturalist Charles Darwin sketched out his theory of evolution in the 1859 book On the Origin of Species – proposing that biological species change over time through the acquisition of traits that favour survival and reproduction – it provoked a revolution in scientific thought.

Now 164 years later, nine scientists and philosophers on Monday proposed a new law of nature that includes the biological evolution described by Darwin as a vibrant example of a much broader phenomenon, one that appears at the level of atoms, minerals, planetary atmospheres, planets, stars and more.

It holds that complex natural systems evolve to states of greater patterning, diversity and complexity.

Also Read | Bottleneck in human evolution explained using a novel genomic analysis technique

“We see evolution as a universal process that applies to numerous systems, both living and nonliving, that increase in diversity and patterning through time,” said Carnegie Institution for Science mineralogist and astrobiologist Robert Hazen, a co-author of the scientific paper describing the law in the journal Proceedings of the National Academy of Sciences.

Titled the “law of increasing functional information,” it holds that evolving systems, biological and non-biological, always form from numerous interacting building blocks like atoms or cells, and that processes exist – such as cellular mutation – that generate many different configurations. Evolution occurs, it holds, when these various configurations are subject to selection for useful functions.

“We have well-documented laws that describe such everyday phenomena as forces, motions, gravity, electricity and magnetism and energy,” Hazen said. “But these laws do not, individually or collectively, describe or explain why the universe keeps getting more diverse and complex at scales of atoms, molecules, minerals and more.”

In stars, for instance, just two elements – hydrogen and helium – were the main ingredients in the first stellar generation following the Big Bang about 13.8 billion years ago that initiated the universe.

That first generation of stars, in the thermonuclear fusion caldrons at their cores, forged about 20 heavier elements such as carbon, nitrogen and oxygen that were blasted into space when they exploded at the end of their life cycles. The subsequent generation of stars that formed from the remnants of the prior generation then similarly forged almost 100 more elements.

On Earth, living organisms acquired greater complexity including the pivotal moment when multicellular life originated.

“Imagine a system of atoms or molecules that can exist in countless trillions of different arrangements or configurations,” Hazen said. “Only a small fraction of all possible configurations will ‘work’ – that is, they will have some useful degree of function. So, nature just prefers those functional configurations.”

Hazen added that “function” might mean that a collection of atoms makes a stable mineral crystal that can persist, or that a star maintains its dynamic structure, or that “a life form learns a new ‘trick’ that allows it to compete better than its neighbors,” Hazen added.

Also Read | Six recent discoveries that have changed how we think about human origins

The authors proposed three universal concepts of selection: the basic ability to endure; the enduring nature of active processes that may enable evolution; and the emergence of novel characteristics as an adaptation to an environment.

Some biological examples of this “novelty generation” include organisms developing the ability to swim, walk, fly and think. Our species emerged after the human evolutionary lineage diverged from the chimpanzee lineage and acquired an array of traits including upright walking and increased brain size.

“I think this paper is important because it describes a view of the cosmos rooted in function,” said Carnegie Institution astrobiologist and planetary scientist Michael Wong, the paper’s lead author.

“The significance of formulating such a law is that it provides a new perspective on why the diverse systems that make up the cosmos evolve the way they do, and may allow predictions about how unfamiliar systems – like the organic chemistry on Saturn’s moon Titan – develop over time,” added co-author Jonathan Lunine, chair of Cornell University’s astronomy department, referencing a world being scrutinized for possible extraterrestrial life.