What happens when scientists start changing the brain – the very system through which an animal’s mental states are expressed in behaviour?

In a recent study published in Nature, researchers implanted human brain organoids – small, simplified structures grown from human stem cells – into the brains of living mice. Over time, these organoids developed and integrated with the mouse nervous system.

Transplanting organoids composed of human cells into animals, including their brains, is not new. However, previous experiments faced an obstacle: human brain cells grow much more slowly than mouse brain cells, and so organoids are quickly “out-competed” before they can fully integrate with the mouse brain. In the new study, scientists genetically altered mice so that parts of their brain did not develop, then transplanted the human organoids into the vacant space.

The idea of creating human-animal chimeras – animals containing human cells – raises ethical questions, particularly when involving the brain. Various philosophers have argued that as the proportion of human brain tissue increases, we should consider whether the animal could acquire human-like mental capacities – and whether this should change how we treat it. But I argue that serious questions about our ability to recognise animal suffering will arise long before mice gain increased intelligence.

‘Super-intelligent’ mice?

What is it like to be a mouse?

The philosopher Thomas Nagel asked a version of this question about bats. He argued that conscious beings have a specific experience of the world, but that others cannot access that experience from the outside. Instead, we must infer what others experience. With animals, we do this through their behaviour and physiology. But this approach assumes that we understand how what we observe relates to what the animal experiences.

There is nothing morally important about a cell simply because it came from a human. Imagine two mice with identical mental lives and cognitive abilities, one produced using transplanted human cells and the other through a modification involving only mouse cells. It is difficult to see why the first should matter more simply because some of its cells are human.

If chimeras pose a moral problem, it is more plausibly because of changes to their cognitive capacities, than because they blur a species boundary.

There are good reasons to doubt that a mouse containing human brain cells could develop anything resembling human intelligence. This is because human-like cognition does not emerge simply from having enough human neural cells. A mouse containing human cortical neurons still has mouse sensory systems, a mouse body and develops according to species specific processes. All of these factors impose constraints on the kinds of cognitive capacities it might acquire.

Animal welfare science depends on interpretation. A mouse cannot report that it is stressed or in pain, so researchers rely on behavioural and physiological indicators. Changes in nesting, social interaction or facial expressions associated with pain, for example, can provide evidence about mouse welfare.

Changing an animal’s brain complicates these interpretations. This is because it alters the very biological system responsible for generating an animal’s experiences, and for producing much of the behaviour through which we infer those experiences.

A mouse might still seem to behave “like a mouse” even as we become less certain that its behaviour accurately tells us how it feels.

In the recent Nature study, researchers reported that mice which received the organoid implant performed on a par with healthy control mice on short-term memory tasks (such as navigating a maze). The researchers did not observe any evidence of altered behaviour or increased intelligence.

This is reassuring. But suppose a chimera grooms less than an ordinary mouse. Researchers can still measure that difference accurately, but interpreting it as evidence of distress means we’re assuming that grooming retains roughly the same relationship to the animal’s experiences as it does in an ordinary mouse.

The same applies to the observation of “normal” mouse behaviour. Since we don’t know exactly how changing the brain will change behaviour, this could make it harder to recognise if the indicators of poor welfare we rely upon are no longer accurate.

What follows for research?

Organoids grown in a dish have been important tools for studying human brain development and disease. But a dish cannot reproduce the environment of a living brain. Transplantation allows researchers to study human brain cells as they develop within a functioning nervous system, potentially providing better models of brain disorders and new ways to test treatments.

This does not, by itself, settle the debate over whether research using chimeras should stop. Uncertainty about an animal’s welfare does not automatically mean that its welfare is worse. It could, in principle, be better. Abandoning potentially valuable research also carries a cost.

Proposals researchers have made for overseeing studies involving chimeras already emphasise careful behavioural monitoring. But this may not be enough. Researchers may also need to establish that the indicators they measure continue to tell us what we think they do. As neural interventions become more extensive, researchers may therefore need to re-validate their methods of welfare assessment.

What happens if we eventually create an animal with human-like intelligence or consciousness? We may never reach that point. But we do not need to reach it before chimeras present a difficult ethical problem. Long before we create an animal that thinks like us, we may create one whose behaviour we no longer understand.

Mackenzie Graham receives funding from The Kavli Foundation, and the National Institute for Health Research.

My recent discovery of a Roman fort and its extraordinary connection with medieval folklore was the fruition of many hours of painstaking research.

This all began in 2002 when I discovered or, more accurately, rediscovered a Roman villa some 30 kilometres to the southwest of the fort. The site was first identified in the early 19th century by Richard Fenton, a local antiquarian. For several reasons, his reliability has been questioned, but mostly because of the perception that the Romans did not reach west Wales was rife in the Regency period and persists to this day.

When my team and I were digging the villa, a local volunteer asked what a Roman site of this status was doing in the middle of nowhere. This was a good question, and I did not know the answer. However, the archaeologist Sir Mortimer Wheeler dug a similar site on the same latitude in 1922, and I reasoned that both sites could lay close to a Roman road. This began a quest to trace a network of routes through west Wales.

A stretch of Roman road was in fact identified by Fenton in the vale of Whitland. This was excavated in 1995, in advance of the construction of a bypass, but I felt that there was more of a network to be discovered. I made breakthrough in 2022. Several other Romano-British sites have been discovered since Wheeler’s day, including a fort at Wiston near Haverfordwest in 2012–2013, and several villas.

Like Fenton, my observations have attracted a good deal of scepticism, but my heart and mind felt that I was making considerable progress tracing the Roman road network from Pembrokeshire east into Carmarthenshire.

My theory received a boost in 2024 with the observation of earthworks suggestive of a Roman auxiliary fort adjacent to an almost certain Roman road near Crymych in north Pembrokeshire. The earthworks had a characteristic playing-card shape like auxiliary forts and was about three hectares in area, which is typical for this type of military installation.

One corner has been destroyed by the construction of a post-medieval settlement (the village of Crymych), which is still inhabited today. The south of the enclosure is intersected by a leat (aqueduct) which enters the site through a masonry channel. Every Roman fort of course needed a water supply. The site lies adjacent to the A478, which runs straight in that area, and many sections further south are similar, but the line is lost in places which is often the case with Roman roads.

It is my belief that it is part of a previously unidentified major arterial Roman road connecting modern Pembrokeshire with Ceredigion to the north.

My hypothesis gained traction with the results of a limited geophysical survey in October 2025, which placed two rectilinear (straight-lined and right-angled) buildings in the right place. Trial excavations followed in March and July 2025 and a steady trickle of finds followed, including coins, a gaming counter, some Roman pottery and a spindle whorl for spinning wool. A more comprehensive evaluation excavation was needed, however, to establish for certain whether the site was a Roman auxiliary fort.

Connections to Welsh mythology

When I discovered the site in 2024, I began to think that it may relate to Cadair Maxen, the camp of the Roman emperor Magnus Maximus (Macsen Wledig in Welsh). He’s mentioned in the Mabinogion (a collection of medieval Welsh stories) in The Tale of Emperor Maxen, also known as The Dream of Macsen Wledig.

This places the late 4th century usurping emperor on the Frenni Fawr, the easternmost peak of the Preseli Hills, which dominates the site a little over three kilometres to the north-east. However, there are no visible remains of a Roman marching camp on the Frenni Fawr where the emperor is said to have visited.

Our breakthrough came in early September with the excavation of five trenches. Three across the suspected principal buildings (the commander’s house headquarters building and granaries); one in the area to the north where a perimeter road would be expected; and another up-slope where the geophysical survey (non-invasive ground scans detecting buried stone structures and drainage ditches beneath the soil) indicated a track.

In the first and second trenches, we identified the bases of floor structures built of clay and packed with the remnants of thin stone blocks. Many of the latter had been disturbed by intermittent ploughing over many decades.

In the second trench there was a diagonal alignment of stones, which indicated a later phase of civilian occupation. We recovered many pottery sherds, particularly in the second trench and found a kiln which yielded many fragments of Roman ceramic tile.

The third trench had a clay floor consistent with Roman granaries. In the fourth trench we unearthed the remnants of the rampart base and a road surface below plough level with rut marks on their expected orientation. The fifth trench did indeed reveal a track. Its date has yet to be established, but we did recover a small number of pottery sherds which are seemingly Roman.

Other new finds from the site include a lead spindle-whorl, an iron stylus for writing documents and letters on wax tablets, a woodworking axe, a gaming counter, a third-century coin and two later fourth century coins.

The collective findings indicate that the site was once a typical timber fort, probably established during the campaign of the governor Sextus Julius Frontinus between AD74 and 78. It does indeed suggest a connection between the camp of Magnus Maximus mentioned in the Mabinogion and the site I first identified in 2024. Magnus Maximus will feature in my forthcoming book, Lives of the Roman Emperors: From Augustus to Romulus Augustulus.

The earthworks are still visible today, almost certainly adjacent to a major Roman road, and would have been more conspicuous in the 4th century and listed on official itineraries. The discovery represents an extraordinary fusion between myth and reality and demonstrates that folklore is often underpinned by fact.

Mark Merrony does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

The question of when animals first evolved is one that has confounded scientists since the time of Charles Darwin. Studying rocks in north Wales in the mid-19th century, Darwin found the sudden first appearance of animal shells and skeletons in layers dating back over 500 million years perplexing. He had predicted a long history of evolution, with animals slowly gaining complexity through time, rather than this apparent instantaneous appearance of complex animals.

The sudden appearance of complex animals has led scientists to argue that animals must have a “hidden history”, a period in Earth’s past prior to their fossil record where they existed but have left no trace of their presence in the rocks. Our new study may help scientists understand how long this hidden history may have lasted.

Palaeontologists have made significant progress since Darwin roamed north Wales. The oldest animal fossils are now recognised in rocks 574 million years old from the UK and Canada. Although these fossils do not have hard skeletons, they are nevertheless visible to the naked eye and relatively complex just like Darwin’s Welsh fossils. Scientists have pushed the date of the first animal fossils back in time, but researchers still haven’t resolved why these oldest remains of animal life are so complex.

Molecular clock studies aim to tackle this problem. A molecular clock works like the speed = distance/time equation. The genetic difference between two living species’ DNA provides the “distance”. If we can estimate the “speed” at which those differences accumulated, we can work backward to estimate the “time” at which their ancestors separated. Fossils provide important dated anchor points that help calibrate the “speed”.

In some instances, such as the divergence of humans and chimpanzees, (when they split from a common ancestor and became different species) molecular clocks can be incredibly effective at estimating the timing of evolutionary origins. However, in the case of animal evolution, molecular clocks give wildly different answers. Depending on how the clocks are calibrated, estimates for the first animals can be hundreds of millions of years apart. Some suggest animals evolved 800–700 million years ago, but others indicate that they evolved only around 600 million years ago. In short, scientists don’t know when animals first evolved.

With colleagues, we set out to address this issue in our recent paper. Much of the disagreement between molecular clocks is due to the different fossil calibrations used in the models. For example, 590-million-year-old rocks in China, which have the conditions to preserve early animals as fossils, but don’t show any evidence of animal fossils, have been used to suggest animals hadn’t evolved by this time.

We show that approximately 550-million-year-old rocks from Mongolia also have these same fossilisation-promoting conditions but still do not preserve animals. The problem is that animals have been found all over the world in rocks 550 million years old. Animals existed elsewhere at the same time. Their local absence in Mongolia might be because they didn’t live in that place or were not preserved. An empty fossil deposit like the one in Mongolia can tell us that animals are absent from that deposit, but not necessarily that animals were absent from the entire planet.

Earth’s deep past

The same could be true for the 590-million-year-old rocks in China. Perhaps animals are not preserved in them because they simply didn’t live there or were rare at the time. This casts doubt on our ability to use the Chinese rocks to argue animals were yet to evolve 590 million years ago.

We argue that palaeontologists need to use several deposits of roughly the same age from around the world, with each demonstrating the right conditions for animal fossilisation but producing no fossils. Only then might we have more confidence in the result.

We suggest that there might be enough deposits meeting these criteria around 900 to 720 million years ago to justify calibrating the animal molecular clock in this interval. Even then, we think that we could only do so with around 70% confidence (and a 30% chance that our evidence of absence is wrong with animals already having evolved).

Using these older rocks suggests that animals originated between 850 and 715 million years ago. Intriguingly, this older age revitalises the idea that animal evolution coincided with snowball Earth, when ice may have covered much of the planet.

The timing of animal origins remains uncertain. But by learning how to read not only the fossils preserved in rocks, but also the fossils that are missing, we are beginning to bring the hidden early history of animal life into view.

Ross Anderson receives funding from the Royal Society.

Orin Lole Durbin does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

For more than a century, we could see Florence Buchanan’s science but not Florence Buchanan. Her papers and electrical traces survived, and her work placed her alongside some of the most influential physiologists of the early 20th century. Yet until recently, no authenticated portrait of Buchanan was known.

Our recent research has now recovered that missing face. There is an irony here: Buchanan used sensitive recording instruments to turn the tiny electrical changes accompanying muscle activity into visible traces. These recordings allowed her to measure events too brief for the eye to follow, helping her investigate how signals travelling through nerves produce a muscular response.

In 1908, Buchanan published a study of spinal reflexes in frogs. It was the opening paper in the first volume of The Quarterly Journal of Experimental Physiology. Using a capillary electrometer, an early electrical recorder that turned tiny voltage changes in muscle into photographic traces, Buchanan could distinguish an immediate muscle response from a later reflex that had travelled into and back out of the spinal cord.

After allowing for the time taken by signals travelling along peripheral nerves, she estimated that a frog’s spinal cord introduced a delay of roughly 12 to 22 milliseconds. Her measurement captured the overall delay produced within the spinal cord during the reflex. Later research was able to separate the different processes contributing to that interval, showing that transmission across an individual synapse, the junction where one nerve cell passes a signal to another, could take far less time than the reflex as a whole.

Buchanan’s experiments therefore provided early evidence that the spinal cord did more than carry signals between the body and brain. Information was being processed within the nervous system itself, an idea that became fundamental to understanding how networks of nerve cells receive, modify and pass on information.

Contribution without belonging

Buchanan was also making herself visible in a scientific world built largely for men. In 1896 she became the first woman known to attend a meeting of The Physiological Society. But attendance did not necessarily mean belonging. She was not allowed to join the society dinner, one of the informal spaces where scientific relationships and reputations were forged.

These dinners were far more than social occasions. They were the centrepiece of the society’s meetings, and routinely included live animal experiments and physiological demonstrations, reflecting the hands-on experimental culture of the period. Ernest Starling, one of Britain’s leading physiologists and a prominent member of the society, captured the attitudes of the time with an extraordinary objection to admitting women: “it would be improper to dine with ladies smelling of dog, the men smelling of dog that is”.

In July 1915, Buchanan became one of the society’s first six women members, finally receiving the same formal rights, duties and privileges as men. The rules were changing, but the culture would take much longer to follow.

Institutional barriers were not the only constraints Buchanan learned to work around. Her sister Lilian Adam Smith’s privately published 1941 memoir, The Buchanan Family, records that Buchanan’s eyesight progressively deteriorated, apparently because of retinal detachment. In 1901, it became so severe that she was ordered to spend three months lying flat on her back. Unable to write, she dictated to a friend an entire 66-page scientific paper on the electrical activity of muscle, an extraordinary feat of memory and concentration.

As her sight failed further, she adapted again. Her laboratory was kept sufficiently unchanged for her to navigate it through memory and touch, and she documented her own visual symptoms in the hope that they might help others. She ultimately donated her eyes for scientific study.

She continued cycling despite the alarm of relatives. When urged to stop, she replied: “Do not ask me to give up the one bit of adventure that is left in my life.” Despite all we knew about her science and character, however, we still had no face by which to remember her.

Finding Florence

Faced with that visual absence, in 2026 I created a provisional AI-assisted reconstruction of how Buchanan might have appeared. It was explicitly presented as an evidence-informed visual hypothesis, with no claim that it represented her recovered face. AI tools were later used more narrowly to help widen the search for an authentic image, suggesting additional terms and combinations to try in archival catalogues. The discovery itself, however, came through conventional archival research: searching collections and following the documentary trail until we found her.

Brian Clark found a securely captioned portrait in the same 1941 family memoir by Buchanan’s sister. The book, held by the National Library of Scotland, was primarily a record of the Buchanan family and their famous father, public-health physician Sir George Buchanan. Florence had not vanished from the archive. In a sense, she had been filed under a man.

The photograph shows a bespectacled woman in profile. “Florence Buchanan” appears beneath it, the memoir’s list of illustrations identifies the plate, and the portrait sits opposite her sister’s remembrance of her. The discovery did more than supply a missing face. It reconnected the scientific record with the woman who created it.

This story reaches far beyond physiology. Women working in science today inhabit a very different world from Buchanan’s, but unequal visibility and recognition have not disappeared. Unesco reports that only around one in three researchers worldwide is a woman, with scientific leaders even more poorly represented. A study of 9,778 research teams likewise found that women were less likely than men to receive authorship or inventor credit for work produced by their teams.

Scientific memory, after all, is built from more than just data. It is also built from portraits, prizes, biographies, lecture slides and the people repeatedly held up as the faces of discovery. Who we see shapes our sense of who belongs.

Florence Buchanan spent her career turning fleeting electrical events into permanent traces. Those traces survived for more than a century, yet her portrait remained overlooked. Now, at long last, the science and the scientist can be seen together.

D.M.B. is supported by a Royal Society Wolfson Research Fellowship (Grant No. WM170007) and is the current Editor-in-Chief of Experimental Physiology, the successor to The Quarterly Journal of Experimental Physiology.

Frances Ashcroft received no funding associated with this article. In the past she has received funding for research on insulin secretion in health and disease from numerous bodies including the Wellcome Trust, the BBSRC, the MRC, Diabetes UK, the European Research Council and the Royal Society.

Brian Clark does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.