This copper plate was discovered in the late 19th century in a prehistoric Native American burial at the Etowah Indian Mounds archaeological site in Georgia, but it was likely made hundreds of miles away at Cahokia Mounds in Illinois. While the decorative plate showcases long-distance trade and a shared ritual culture among early Indigenous groups in the southeastern United States, the nature of the human figure depicted on the artifact has been debated for over a century.

The "Birdman" relic is one of two hammered copper plates that Smithsonian archaeologist John P. Rogan unearthed in the 1880s while digging in a burial mound at Etowah, a centuries-old village occupied by thousands of Native Americans from 1000 to 1550. Etowah was part of the "mound-building," or Mississippian, phase of Indigenous occupation; other famous sites include Moundville in Alabama and Cahokia in Illinois. These groups had extensive trade networks and shared a common artistic and ceremonial tradition known as the Southeastern Ceremonial Complex.

This copper artifact, known as Rogan Plate 1 or the "Birdman" plate, is in the collection of the Smithsonian National Museum of Natural History in Washington, D.C. The plate is roughly 11 inches (28 centimeters) tall and had been buried with a high-status person who likely died at Etowah in the mid-14th century. But experts believe the copper plate was an heirloom made decades earlier, at Cahokia, and traded south to Etowah.

A human figure dominates the copper plate with its elaborate bird costume. The person carries a severed head by the hair in their left hand, while their right hand holds a club or mace. The figure also wears a pleated skirt or loincloth that may have been made from human scalps. This bird-human figure is commonly depicted in the Southeastern Ceremonial Complex and has been called a dancing warrior, eagle warrior, eagle dancer, hawk dancer, falcon dancer and falcon warrior. But more often, the figure is called Birdman or Morning Star, a prehistoric Mississippian hero associated with agriculture and rebirth.

Archaeologists' interpretations of the Birdman plaque from Etowah have long assumed that the represented figure is a male warrior. In a 2004 study, for example, archaeologist Adam King wrote that "this plaque would have served as an emblem of office awarded to a chieftain, and would have accompanied him as a sign of his strength among the ancestral spirits."

But the sex of the skeleton in the Etowah burial is unclear, and there were high-status graves of both men and women at Etowah, archaeologist Billie Follensbee and art historian Katie Buford wrote in a 2017 study. Although experts previously assumed that both Rogan Plate 1 and Rogan Plate 2 represented male warriors, Follensbee and Buford found that the figures have slender bodies with prominent right breasts, suggesting the "Birdman" may have actually been female.

"A female bird-human warrior could have represented a gender variant individual in the form of the well-known Native American phenomenon of the 'manly-hearted woman,' a woman who has assumed the usually male role and the garb of a combat soldier," Follensbee and Buford wrote. In addition to the historic term "manly-hearted woman," third-gender or gender-variant individuals in Indigenous North America are often referred to as "two-spirit," a broad term Indigenous people coined in 1990 to describe people who take on gender-nonconforming roles.

The commemoration of a two-spirit individual on a copper artifact, as well as the use of the artifact as a high-status grave good, "not only demonstrates the Native American cultural acceptance of such gender variant roles but also further serves to celebrate the elite status and achievements of these 'manly-hearted women,'" Follensbee and Buford wrote.

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Hotter weather is expected to pose dangerous health risks for millions, and pregnant people are especially vulnerable to its impacts, experts caution.

"Pregnant people are physiologically more vulnerable to heat," Dr. Susanna Esposito, a pediatrician at the University of Parma in Italy, told Live Science. "Climate adaptation should treat pregnant people as a priority population."

Extreme heat is linked to adverse birth outcomes that can be fatal for both mothers and newborns. Heat can also hinder fetuses' development, contributing to long-term health and cognitive effects.

Scientists and public health experts are working to identify the thresholds between safe and dangerous heat for maternal and prenatal health. As the planet warms, how will that affect the risks associated with pregnancy? And how can those risks be countered?

Low birth weight

When the body tries to cool down, blood moves to the extremities to dissipate heat through the skin's surface. Some studies suggest that, in a pregnant person, this may reduce blood flow to the placenta, which in turn reduces the organ's supply of oxygen and nutrients. This is thought to contribute to low birth weight, which is associated with a heightened risk of illness and death shortly after birth, as well as cognitive and mental-health challenges later in life.

Heat stress may also spike the body's levels of cortisol beyond what they should be in a healthy pregnancy, studies in lab animals suggest. This may expose developing fetuses to more cortisol than usual, affecting some aspects of development and functions of the placenta. This mechanism is not fully understood in humans yet, though.

That said, several studies in humans support the notion of a link between heat exposure and low birth weight. Mothers who were pregnant during a 2003 heat wave in France were more than twice as likely to have a baby of low birth weight than mothers whose pregnancies did not coincide with the extreme heat. The trend was especially strong when the heat wave hit during a person's first trimester.

In a study conducted in Arizona, hotter temperatures during the third trimester were also associated with lower birth weight, said study co-author Kristin Mickelson, a sociologist who studies public health at the Arizona State University.

Preterm and early birth

Preterm delivery — before 37 weeks of gestation — is one of the most concerning health outcomes that's been linked to heat, Esposito told Live Science. The risk of neurological and developmental disabilities is higher in preterm babies than in full-term babies.

"If you're born preterm, you have a heightened risk of all sorts of adversities throughout infancy, but then also extending into childhood and even longer term," said Clara Heil, health researcher at Lund University in Sweden.

Severe heat may make preterm birth more likely. A review of 70 studies suggests that the chance of preterm birth is 16% higher when a multiday heat wave hits during the final weeks of pregnancy. Regarding average temperatures across an entire pregnancy, each additional 1.8 degrees Fahrenheit (1 degree Celsius) boosts the risk of preterm birth by 5%, the same review found.

High temperatures have also been tied to early births, which occur after 37 weeks of pregnancy but before 39 completed weeks. Early birth is associated with an increased risk of hospitalizations through childhood and slower neurologic development.

A study analyzing 56 million U.S. births across two decades suggested there was about 1 more early birth per 100,000 pregnancies on days with a maximum temperature above 90 F (32.2 C), compared to 60 to 70 F (15 to 21 C). Deliveries on hot days were typically about six days earlier than anticipated, but some were up to two weeks early.

With average temperatures continuing to rise, early and premature births could become more common. The United Nations predicts that the world is set to cross the 1.5 C (2.7 F) threshold of warming compared with pre-industrial levels, likely within the next five years, and we may even surpass the 2 C (3.6 F) threshold in that timeframe.

"We can probably expect to see these temperatures, or worse, in the upcoming years," Lizbeth Burgos Ochoa, statistician at Tilburg University in the Netherlands, told Live Science.

Preeclampsia, heart defects and other risks

Maternal heat exposure has also been associated with an increased risk of preeclampsia, a pregnancy complication related to blood pressure that can be fatal. In a 2011 study conducted in France, hotter temperatures during the first trimester were associated with a slightly increased risk of severe preeclampsia, although more research is needed to understand why.

Heat exposure has also been linked to other conditions of pregnancy, including gestational diabetes and hypertension, as well as increased emergency hospital admissions during pregnancy, overall.

Additional studies have highlighted potential risks for developing fetuses and newborns. For example, following a heat wave in Canada, the risk of a baby being born with critical heart defects was 6% higher, one study found. Babies with the highest risk had been exposed to temperatures of 86 F (30 C) or higher for at least 15 days during the first trimester of pregnancy.

Extreme heat waves may also raise the chance of stillbirth, according to a study conducted across six U.S. states. Four consecutive unusually hot days — exceeding the 97.5th percentile of temperature for a given county — in the last week of pregnancy was linked to a 3% increase in the risk of stillbirth.

These studies are observational, so they show links between heat and pregnancy outcomes but cannot directly show that the heat spurred the increased risks. They also don't take into account study participants' home environments or strategies for cooling off, so they don't provide a full picture of how much heat they were exposed to.

Caring for pregnant people and the next generation

Researchers are still working to understand what temperatures pregnant people should be most concerned about.

"It would be [helpful] to understand what level of heat becomes dangerous to women," Heil said. "Is there a threshold where it becomes difficult for the body to dissipate heat? And especially for the pregnant body, which is different from the non-pregnant body, to cope with that heat?"

Heil and her research team synthesized over 100 studies on heat and poor birth outcomes, such as stillbirth, preterm birth, placental abruption and preeclampsia. The studies varied considerably in that they covered different timeframes, focusing on just one trimester or all three, for instance. But in general, their results converged on a pattern: The risk of poor outcomes rose as temperatures hit about 87 F (30.5 C). At 102 F (39 C), the combined risk of poor outcomes rose by 20%.

This summer, daytime maximum temperatures in the continental U.S. averaged a record-breaking 88.6 F (31.4 C). The number of days per year over 105 F (35 C) is expected to triple in the U.S. by midcentury.

Heat's detrimental effects on birth outcomes are even more extreme for those with disadvantaged socioeconomic conditions, Burgos Ochoa noted. People with lower incomes are less likely to have air conditioning, and neighborhoods with a lower median income tend to have less green space, which contributes to trapped heat in their immediate surroundings. Expanding green spaces and adding bodies of water can help cool down neighborhoods and mitigate heat impacts on health, she said.

Heil's research team has developed a phone app for pregnant people and those who have recently given birth that forecasts the risk of heat stress and suggests actions to take to mitigate those risks, such as hydrating or changing clothing. Heil is hopeful that the app and similar early-warning systems can make a difference.

"Maternal health should be at the top [of our concerns], because it is about the next generation," Mickelson said. "If we don't take care of the mothers that are giving birth to the next generation, that's going to have long-term consequences down the road."

This article is for informational purposes only and is not meant to offer medical advice.

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Glaucoma, which affects about 80 million people worldwide, is a group of diseases that damage the optic nerve, which carries visual information from the eye to the brain. It can lead to permanent vision loss, usually affecting peripheral vision at first and later central vision. Cataracts are even more common, affecting more than 94 million people globally; the condition occurs when the eye's lens becomes cloudy, causing blurred vision and, eventually, blindness.

Aging raises the risk of both diseases, as do certain medical conditions, such as diabetes. But now, a review of 41 studies suggests that people who are exposed to higher levels of air pollution may also face a greater risk of these diseases. The research, presented in September at the 44th Congress of the European Society of Cataract and Refractive Surgeons in London, brought together evidence from dozens of studies collectively involving millions of people across East Asia and the U.K.

The studies were observational, meaning they can only draw a link between air pollution and the eye conditions, rather than prove that air pollution causes the diseases. But still, "this reframes glaucoma and cataract as conditions shaped by where you live and breathe, not just how long you've lived," said study co-author Dr. Ahmed Alnabihi, an ophthalmologist at King Khaled Eye Specialist Hospital and Research Center in Saudi Arabia, who presented the findings. These environmental exposures may compound age-related decline in the eye as they do in cardiovascular disease, he said.

PM2.5 shows the clearest links

The researchers examined exposure to several pollutants, including different sizes of particulate matter (PM), such as fine PM2.5 and larger PM10. They also looked at nitrogen dioxide, sulfur dioxide, ozone and carbon monoxide.

Most studies included in the review estimated people's long-term exposure based on the pollution levels around their homes and in their communities. They used a range of methods, including government monitoring systems, satellite data and computer models designed to estimate pollution levels. A few studies examined short-term exposure and its relation to more sudden eye issues, such as acute angle-closure glaucoma, a medical emergency that can quickly cause vision loss.

Using this wealth of data, the team compared pollution exposure with rates of glaucoma, cataracts and age-related macular degeneration (AMD), another common eye condition that impairs vision. PM2.5 — tiny particles that measure 2.5 micrometers or smaller — showed the clearest associations with glaucoma and cataract risk.

People with the highest exposure to PM2.5 had 70% higher odds of glaucoma, compared to those with the least exposure. Plus, an increase in PM2.5 exposure equivalent to the difference between the 25th and 75th percentiles of exposure within each study was associated with 8% higher odds of glaucoma and a 4% higher hazard of cataracts.

The evidence was less consistent for air pollution and age-related macular degeneration, which damages the central part of the retina that's responsible for sharp, detailed vision. And unlike the clear connection seen with PM2.5, other pollutants showed less consistent links to eye disease. For instance, nitrogen dioxide was not significantly tied to AMD, ozone was not tied to cataracts, and neither pollutant was linked to glaucoma. Sulfur dioxide and PM10 showed no significant associations with any of the three conditions.

The association between PM2.5 and glaucoma was relatively consistent across the reviewed studies, said Nidhi Singh, an environmental epidemiologist at the IUF-Leibniz Research Institute for Environmental Medicine in Germany, who was not involved in the study. The findings for PM2.5 and cataracts were moderately consistent, albeit more variable than the glaucoma data; still, she said that the analysis seems to support a link.

Singh did caution that the underlying evidence is observational. "Therefore, the results demonstrate an association, not by themselves a causal effect of PM2.5 on glaucoma or cataract," she told Live Science.

What explains the increased risk?

The exact biological mechanisms behind these links remain unclear, but Alnabihi noted that fine particulate matter is small enough to be absorbed into the body. This can trigger oxidative stress, in which harmful molecules overwhelm natural defenses and cause damage. It also drives chronic, low-grade inflammation, in which the immune system stays mildly activated for long periods, he explained.

In the eye's lens, these processes can "damage the crystallin proteins that keep it transparent, causing them to misfold and cloud, which is what a cataract is," Alnabihi said.

In the optic nerve, the same processes are thought to damage retinal ganglion cells, a special type of neuron that relays information from the retina to the brain. This can contribute to glaucoma independent of internal eye pressure, he said, which is another big risk factor.

"Unlike the lungs or gut, its [the eye's] surface is in direct, continuous contact with outdoor air, giving pollutants a uniquely direct route that might cause damage," he added.

Other studies have found clues that support the link between PM2.5 and eye damage. Mice that were exposed to ambient PM2.5 for nine months developed elevated pressure inside their eyes, a glaucoma risk factor. The researchers observed both oxidative stress and nitrative stress — a similar phenomenon — in the tissues that drain fluid from the eye, which may help to explain the rise in pressure.

Studies of humans and human cells have uncovered similar patterns. In data from more than 480,000 people in the UK Biobank, a large repository of health information, those exposed to higher PM2.5 were more likely to develop glaucoma over time. In another study, scientists exposed lab-grown human retinal tissue made from stem cells to PM2.5. The particles disrupted retinal cell development, including that of retinal ganglion cells, while increasing cell death.

These studies offer pieces of a possible mechanism, but for now, how those pieces fit together in people remains unclear.

This article is for informational purposes only and is not meant to offer medical advice.