When the final of the 2026 men’s football World Cup kicks off in New Jersey this weekend, players can expect temperatures of over 30°C (86°F). Indeed, much of the tournament has been played in challenging conditions – like the high elevation and thin air of Mexico City’s Estadio Azteca – giving pundits plenty to chew over.

But one factor has been less discussed: humidity. High temperatures may be uncomfortable, but it is really the combination of heat and high humidity that can make a football match – indeed, any activity – physiologically unbearable. Humid conditions make it harder for our bodies to benefit from sweating, as the air is already laden with moisture, hampering our ability to cool down.

This impact of humidity is still underappreciated. One way of taking both humidity and heat into account is the wet bulb globe temperature (WBGT), a measure of heat stress used by sports organisations. As 91av went to press, the most extreme match of this World Cup (Uruguay vs Cape Verde) had an estimated WBGT of over 33°C (91°F) – a level at which people are advised to suspend all outdoor activity. Most of us simply aren’t prepared for such conditions, as we explore here.

It is really the combination of heat and humidity that can make activities unbearable

Yet, if we understand the dangers of humidity, we can start to grasp the advantages of dry heat. We detail the many ways in which saunas and heat therapy can keep you healthy, from cardiovascular benefits to protection against Alzheimer’s disease. The key is that saunas operate at extremely low humidity, allowing us to sweat comfortably while reaping the benefits of getting decidedly hot.

Saunas aside, we should still be wary of extreme heat. With European heatwaves on the rise, the 2030 men’s football World Cup – to be played across Portugal, Spain and Morocco – is likely to see low humidity but temperatures of up to 40°C (104°F). With no sign of us cutting carbon emissions, the World Cups of the early 21st century may one day be fondly remembered as nice and cool.

This is an extract from Our Human Story, our newsletter about the revolution in archaeology. Sign up to receive it in your inbox every month.

As someone who writes a lot about human evolution and archaeology, I’ve seen a great many artists’ impressions of prehistoric people. Some are remarkably believable, closely tied to scientific findings as much as possible. Others, not so much. I twitch every time I see a reconstruction of an African or tropical hominin with northern-European-style pale skin, and the twitches escalate whenever I see hairless hominins wandering around naked in temperate regions like Britain. Put something on or you’ll die.

However, the thing that has come to bother me most is that a lot of our images of prehistoric people are just too big.

Sometimes, this is about musculature. You can find pictures showing prehistoric people (generally men) with ripped physiques, muscles bulging, as if they’ve been doing reps at the gym and downing protein shakes. This just isn’t how practical everyday fitness works. If your body looks like a movie superhero’s, that is a cosmetic choice – one that is achieved using physiological trickery, like artificially dehydrating yourself so your muscles stand out. Genuine speed, strength and endurance don’t look like that.

Beyond these surface aesthetics, there is also a deeper issue. Prehistoric people were smaller than us, both in body mass and in stature. At the extreme end of this, we have the truly diminutive groups like Homo floresiensis (known as “hobbits”), but even our direct ancestors were probably, on the whole, on the small side compared with people today.

The story of how humans got heavier and taller is slowly coming together, as our fossil collection expands. And it looks like it was an important part of our origins.

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Small folk

To find out how hominin body size has changed over time, I spoke to evolutionary biologist at the University of Reading in the UK. He’s lead author of a study published in PNAS on 22 June, which looks at .

Let’s start by putting some concrete numbers on this. Earlier hominins like Ardipithecus and Australopithecus, which lived earlier than 2 million years ago, had an average body mass of 40 kilograms, with a range of 30 to 50 kg. Homo habilis, an early member of our genus, was a bit bigger: 45 kg on average, ranging from 35 to 55 kg. Homo erectus, the first hominin known to have migrated out of Africa into Eurasia, was bigger still, averaging 60 kg with a range of 50 to 75 kg. Finally, prehistoric Homo sapiens were up to an average of 75 kg and a range of 55 to 80 kg.

Gardner and his colleagues compiled estimates of body mass for 386 specimens belonging to 21 hominin groups, dating from 4.5 million years ago to 30,000 years ago. If that seems a little arbitrary, there are reasons why. They excluded more recent specimens because there is evidence of our species having shrunk in the past 30,000 years, which would have made a mess of the analysis. They also left out the earliest hominins, Sahelanthropus and Orrorin, because of ongoing uncertainties over how they are related to later groups.

The team then ran 1000 models to see what would best explain the data. Did body size increase within each group, meaning earlier H. erectus were smaller than more recent ones? Or did each group pretty much stay the same size, with the changes coming when new groups evolved? Did body size increase gradually over the whole 4.5-million-year timespan, or were there long periods of stasis and occasional bursts of growth?

The researchers ultimately identified two trends, which between them explained most of the changes.

The first was “the gradual progression from Ardipithecus to later hominin species”, says Gardner. This was a “modest” increase in body mass of about 1 to 3 kg per million years. If you remember, average body mass went from 40 kg to 75 kg in 4.5 million years: this gradual increase can only explain about a third of that.

Hence the second trend, which was a “big jump around the appearance of Homo erectus and other later species in our genus Homo”, says Gardner. The first H. erectus appeared in Africa around 2 million years ago.

We’ll get to why this might have happened in a minute, but first let’s look at what happened to people’s height.

Short kings

Gardner and his colleagues didn’t analyse height, but a team led by at the University of Tübingen in Germany did in . This one compiled 204 stature estimates from 4.4 million years ago until nearly the present day. The variation is dramatic. One especially dainty Australopithecus specimen clocked in at just 105 centimetres, while a 24,500-year-old H. sapiens from Barma Grande cave in Italy was almost 189 cm tall.

Will’s team found “phases of relative stasis intermitted by periods of rapid increases”. Before 2.2 million years ago, most hominins were less than 140 cm tall. There was then a noticeable increase between 2 million and 1.6 million years ago. By 1.6 million years ago, some individuals grew taller than 170 cm for the first time. However, such heights only became commonplace much more recently, around 500,000 years ago.

This loosely maps onto what Gardner’s team found. Hominins got significantly bigger, both in mass and in height, from around 2 million years ago when H. erectus came on the scene.

What was going on? The answer, as usual, is “lots of things”.

One factor may be sexual dimorphism. In early hominins like Australopithecus, males were significantly bigger than females – – and had larger canine teeth. We see the same pattern in chimpanzees and other animals where males compete violently for females, with successful males mating with multiple females while unsuccessful males don’t mate at all. But in early Homo species, males and females were much more alike (and in H. sapiens today, the height and mass differences are smaller still).

Exactly how this played out is unclear. “There’s some hypotheses that perhaps males got smaller through time, and then there’s hypotheses that it’s the females that got bigger through time and the males stayed about the same,” says Gardner. If the latter is true, that would account for some of the average increase.

Diet may also have played a big role. Early Homo ate more meat than Australopithecus, and meat is calorific. Furthermore, “we are finding some of the earliest evidence of cooking meat in Homo erectus”, says Gardner. Cooked food contains more calories than raw food, so this was another way of getting more nutrition from a given food source.

Cooked food may also have been a factor in another important change in our bodies: the growth of our brains. The extra calories from cooked food may have enabled our brains to evolve to be larger.

It’s also crucial to bear in mind that different hominin groups evolved in different ways, depending on their circumstances. So while there was this overall tendency to get bigger, not everyone did. H. floresiensis, who lived on Flores in Indonesia until just 50,000 years ago, were barely more than 100 cm tall. Homo naledi from South Africa, who lived 335,000 to 236,000 years ago, were taller but still not much more than 140 cm. One of the shortest known adult hominins was a female Paranthropus robustus, who was probably just 103 cm. We don’t know why these hominins became so small, and it may be that each one did so for different reasons.

“All these different species are finding their own evolutionary path,” says Gardner. Each hominin group had its own unique set of environmental contexts, and this was reflected in their varying body sizes, he says.

Finally, let’s turn the question around. Once early Homo groups like H. erectus evolved larger bodies, how did that change their lives? “The larger you are,” says Gardner, “the longer the strides you can take across the landscape.” That’s especially true of H. erectus, whose legs were proportionately longer than those of earlier hominins. Gardner suggests they had larger home ranges than previous groups.

This may even help to explain why H. erectus were the first hominins (that we know of) to roam outside Africa. That’s a long-standing puzzle: how come they travelled all the way to Java, when earlier hominins seemingly never left Africa? If Gardner is right, it may be partly because they were bigger, so they needed more room.

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When the sun is out, many of us reach for sunscreen, but myths and misinformation have left some people confused about when and how to use it, and how to ensure we still get enough vitamin D. , a skin cancer expert at QIMR Berghofer Medical Research Institute in Australia, has led clinical trials looking at the link between sun exposure and skin cancer, and sunscreen use and vitamin D. She also recently led the development of a new that considers how to balance the risks and benefits of sun exposure. Here, she lists five things that everyone should know about sunscreen.

Sunscreen should be used as a last line of defence

Many people think it’s OK to lie on the beach all day in a tiny bikini as long as they rub sunscreen all over their exposed skin and reapply it every 2 hours. They think sunscreen makes them bulletproof. But even if you apply the best sunscreen perfectly, it still lets some ultraviolet (UV) radiation through. If you’re out in the sun for hours, that gradually adds up to a dose that is big enough to cause skin damage. At that point, it doesn’t matter if you apply more sunscreen; the damage is already done.

People often assume that if they reapply sunscreen every 2 hours, they’re sort of starting the protection again, but that’s not how it works. You also need to protect yourself with a hat, sunglasses, rash shirt or other protective clothing, and stay in the shade in the middle of the day. Sunscreen should be considered a last line of defence for the parts of your skin that you can’t easily cover while you’re outdoors, like your hands and neck.

There is good evidence that sunscreen protects against skin cancer and wrinkles

The largest, longest-running study of sunscreen was conducted in the Australian town of Nambour. In 1992, 1600 people in the town were randomised to apply daily sunscreen or continue with their normal sunscreen use, which tended to be minimal. It found that those who applied the daily sunscreen were years down the track.

The researchers also created moulds of the backs of the study participants’ hands to look at damage to the surface of the skin. Those in the daily sunscreen group had compared with those who didn’t. When they were followed up on in 2014, they also had .

The sunscreen to choose is the one you like wearing

It’s no good having sunscreen that sits in your cupboard and doesn’t end up on your skin because you don’t like the feel of it. If you’re going on a hike and you’re going to be out all day, it’s better to wear sunscreen with a high sun protection factor (SPF) of 50+. But it’s harder to get a high-SPF sunscreen that feels really nice, so if you’ll be popping out for only short periods throughout the day, you can choose an SPF 15 or 30 sunscreen. Tinted sunscreens can offer the same protection as normal sunscreens, but only if you apply them thickly. But because these often make the skin look overly tinted, people tend to apply them too thinly. One option is to first put on a thick layer of normal sunscreen, then apply the tinted sunscreen on top of it.

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Chemical sunscreens, meaning those that contain organic ingredients such as octocrylene and avobenzone, work by absorbing UV radiation from the sun and converting it to harmless heat. Inorganic sunscreens, also known as mineral or physical sunscreens, contain zinc oxide or titanium dioxide particles. They are often reported to work by reflecting or scattering UV radiation, but they actually , like chemical sunscreens.

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Wearing two layers of sunscreen helps to achieve adequate coverage

You get the SPF listed on the bottle only if you apply 2 milligrams of sunscreen per square centimetre of skin, which is around in an average adult. But it’s really hard to apply this amount of sunscreen in one go. One day, I decided to measure it out exactly, and I couldn’t rub it all on; it was too much. So now, I apply one layer, let it sink in while I brush my teeth and do other things, and then a second layer, so I can apply the full recommended amount.

I was born in Armidale, Australia, in the late 1960s and didn’t wear sunscreen as a child, despite my pale skin. I’ve since had three skin cancers removed, the first of which appeared when I was just 29. So now, I am careful to protect my skin.

If you’re diligent with sunscreen, you might need to take a vitamin D supplement

We recently conducted a trial called the Sun-D Study to see whether applying SPF 50+ sunscreen every day affects people’s vitamin D levels. We randomly assigned 639 people to apply SPF 50+ sunscreen as part of their daily morning routine on days when the UV index was forecast to reach 3 or higher, or to use it at their own discretion. After about a year, a – about 46 per cent compared with 37 per cent in the control group. If you wear sunscreen every day, I would advise taking a vitamin D supplement so that you don’t become deficient, especially in winter. I take one myself – they are cheap, safe and effective.

People with dark-coloured skin are at greater risk of developing a vitamin D deficiency. I recently led the development of a new that looked at how to balance the various risks and benefits of sun exposure. It brought together experts from many Australian universities and medical organisations, and it concluded that people with dark-coloured skin need to put on sunscreen only if they plan to spend more than 2 hours outdoors on days with high UV radiation levels. This is in recognition of the fact that melanoma incidence is 30 times lower in people with dark-coloured skin than in those with light-coloured skin, and that vitamin D deficiency poses a greater risk.

A living organism is made from components that aren’t themselves living. This simple statement has profound implications. For one, it means that there is no mystical force that animates us and other life forms. For another, it means that it should be possible to build a life form from scratch – and we are now a step closer to doing so.

Artificial life has been the guiding light of synthetic biology for some time. In 2010, biologists at the J. Craig Venter Institute in California synthesised the stripped-down genome of a bacterium and inserted it into the chassis of another cell, emptied of its own DNA. The resulting organism, with a record-low number of genes (473) was able to grow and reproduce. But even then, scientists didn’t understand what a third of those genes were doing, or whether they were even needed. Instead of rebooting an existing cell with a synthetic genome, we need to build an organism from the ground up.

That is what scientists at the University of Missouri are now attempting. The SpudCell – named both to evoke Sputnik and the dawn of the space age, and for its resemblance to a potato – is an entity based on just 36 genes. It self-assembled when the genes were supplied with all the building blocks necessary for life, forming cell-like bubbles and making proteins.

SpudCell represents a significant breakthrough in the creation of artificial life

But that’s it. The SpudCell can only make proteins because it is supplied with ribosomes, the crucial cell components that make proteins. It can’t metabolise food, supply itself with energy or reliably divide and reproduce. It isn’t alive, and it needs intensive care just to perform its basic functions. Nevertheless, the SpudCell represents a significant breakthrough in the creation of artificial life. If a modern living cell is a jet airliner, the SpudCell is the rickety wooden-and-cotton proto-airplane made by the Wright brothers.

Better versions will soon follow, with potentially transformative applications. The hope is that synthetic cells will one day be able to supply materials that are currently derived from fossil fuels, such as plastics, fuels and fertiliser. That is keenly needed. But the work in understanding how a living entity operates will shed light on what life needs, and how it emerges from dead materials. If we crack this ultimate mystery, synthetic biology will have really delivered.

The heatwave that swept Europe last week saw many temperature records broken, leading people to ask if extraordinary June heat is the “new normal”. Unfortunately, the truth is that we are never going to have normal in our lifetimes again – just ever more extreme heat.

Climate scientists are continually warning of the need to prepare for hotter heatwaves, worse droughts, more flooding and rising seas. During heatwaves like the one just passed, the hottest and most humid ever seen in Europe, they might even get a little media coverage. But then the weather cools, the news agenda moves on and nothing is ever done.

We are currently on course for average global surface temperatures to rise by between 2.1°C and 3.3°C by 2100, and possibly even more. Even these alarming numbers are a little misleading because the oceans that cover most of the planet don’t warm as fast as the land. Average land temperatures are therefore going to go up by a lot more than the above numbers imply.

But what really matters to us is extreme weather, not the average. The projections for future extremes are already dire, and there are reasons to think that we are in for extremes even greater than those currently projected for a given level of warming. With uncertainties over the survival of the vital AMOC ocean current and the risk of a major glacier collapse, the only thing we really do know is that we must prepare for conditions far worse than Europe has just experienced.

We are on course for average global surface temperatures to rise by between 2.1°C and 3.3°C

It is possible to get through even worse heatwaves if all your infrastructure and systems are geared up to cope, but for most countries, this isn’t the case. The fact is, the world is changing fast and we need to change just about every aspect of our lives to adapt – our homes and offices, factories and schools, cars and trains, farms and gardens, and so on. But it isn’t happening. One day, after the tragic deaths grow too great to bear, we will ask why we did nothing to prevent them.