Humans are getting taller; they're also fatter than ever and live longer than at any time in history. And all of these changes have occurred in the past 100 years, scientists say.

So is evolution via natural selection at play here? Not in the sense of actual genetic changes, as one century is not enough time for such changes to occur, according to researchers.

Most of the transformations that occur within such a short time period "are simply the developmental responses of organisms to changed conditions," such as differences in nutrition, food distribution, health care and hygiene practices, said Stephen Stearns, a professor of ecology and evolutionary biology at Yale University. [10 Things That Make Humans Special]

But the origin of these changes may be much deeper and more complex than that, said Stearns, pointing to a study finding that British soldiers have shot up in height in the past century.

"Evolution has shaped the developmental program that can respond flexibly to changes in the environment," Stearns said. "So when you look at that change the British army recruits went through over about a 100-year period, that was shaped by the evolutionary past."

And though it may seem that natural selection does not affect humans the way it did thousands of years ago, such evolutionary mechanisms still play a role in shaping humans as a species, Stearns said.

"A big take-home point of all current studies of human evolution is that culture, particularly in the form of medicine, but also in the form of urbanization and technological support, clean air and clean water, is changing selection pressures on humans," Stearns told Live Science.

"When you look at what happens when the Taliban denies the polio vaccination in Pakistan, that is actually exerting a selection pressure that is different in Pakistan than we have in New York City," he said.

Here's a look at some of the major changes to humans that have occurred in the past century or so.

(Some) people have grown taller

A recent British study, published by the Institute for the Study of Labor (IZA) in Bonn, Germany, showed that young men in the United Kingdom have grown by 4 inches (10 centimeters) since the turn of the 20th century.

In the study of British recruits, the average height of British men, who had an average age of 20, was about 5 feet 6 inches (168 centimeters) at the turn of the century, whereas now they stand on average at about 5 feet 10 inches (178 cm). The increase can be attributed, most likely, to improved nutrition, health services and hygiene, said the researchers from the University of Essex in Colchester.

In a number of other developed countries, people have been growing taller, too, reaching the world's current greatest average height of 6 foot 1 inch (1.85 meters) in the Netherlands. Interestingly, Americans were the tallest people in the world by World War II, measuring 5.8 feet (1.77 meters), but by the end of the 20th century, they fell behind, and the average U.S. height has stagnated, according to a study by John M. Komlos, currently a visiting professor of economics at Duke University. [Why Did Humans Grow 4 Inches in 100 Years?]

And even in some of those countries where the average height has been rising, the increase has not been uniform. For instance, people from former East Germany are still catching up height-wise with former West Germans after years of communist rule, said Barry Bogin, a professor of biological anthropology at Loughborough University in the United Kingdom. And in some non-Western countries that have been plagued by war, disease and other serious problems, average height has decreased at one point in time or another. For instance, there was a decline in the mean height among blacks in South Africa between the end of the 19th century and 1970, Bogin wrote in one of his studies, published in the Nestle Nutrition Institute workshop series in 2013. He explained that the decline was likely related to the worsening of socio-economic conditions before and during apartheid.

"It shows you the power and the generation-after-generation effects of something bad that happened to your mother gets carried on to you and your children, and it takes about five generations to overcome just one generation of starvation, or epidemic illness, or something like that," Bogin told Live Science.

Unfortunately for those individuals, height seems to improve humans' quality of life and chances of survival. For instance, in the United States, taller people make more money on average, as they are perceived as "more intelligent and powerful," according to one such study published in 2009 in the Economic Record.

Everyone is getting fat

Since the 1970s, Bogin has been studying growth patterns of Maya children and their families living in Guatemala, Mexico, and the United States. When Maya people move to the United States, their kids born here are 4.5 inches (11.4 centimeters) taller than siblings born in Mexico or Guatemala. This likely results from the accessibility of more-nutritious food in the United States, for instance, through lunch programs at schools, as well as better health care, Bogin noted. The Maya kids are also less exposed to infectious diseases, which are less common in the United States than in the countries of the parents' origin. [7 Devastating Infectious Diseases Explained]

But this increase in height comes with a high price tag.

"Not only do these Maya kids begin to look more like Americans in height, but they become even super-Americanized in their weight, by becoming overweight," Bogin told Live Science.

"People are getting fatter everywhere in the world," he said. (In 2013, 29 percent of the world's population was considered overweight or obese, according to a study published May 29 in the journal The Lancet.)

Exactly why humans are getting fatter is currently a question of heated scientific debate. Some researchers point to the traditional argument of eating too much and exercising too little as the culprit, whereas others offer alternative explanations, including the role of genetics and viruses that have been linked to obesity. The issue of excessive weight and obesity gets even more complicated, as many studies have linked being fat with poverty, which goes against a popular association of obesity and wealth.

Interestingly, the Maya kids in Indiantown, Florida, on whom Bogin focused his studies, had the highest rates of being overweight and obese of all ethnic and racial groups in the area, including Mexican-Americans, African-Americans, Haitians and European-Americans. This may have something to do with epigenetics, or heritable changes that turn genes on and off but that are not caused by changes in the DNA sequence. For instance, the environment may have caused epigenetic changes to some ethnic groups that affect how the body stores excessive energy from food, Bogin said.

"There may be an expectation that since your mother suffered and your grandmother suffered, somehow this suffering gets passed on to the current generation of children, and they kind of expect that there is going to be bad times and there is not going to be enough food," he said. "So when there are good times, eat as much as you can, and the body should preferentially store the extra energy as fat."

This mechanism of fat storage driven by a history of malnutrition or starvation may be occurring in other poor populations in the world who are becoming overweight and obese, he said.

Earlier puberty

In many countries, children mature earlier these days. The age of menarche in the United States fell about 0.3 years per decade from the mid-1800s (when girls had their first menstrual period, on average, at age 17) until the 1960s, according to a 2003 study in the journal Endocrine Reviews, which also suggested better nutrition, health and economic conditions often play roles in lowering the age of menarche. Today the average age of menarche in U.S. girls is about 12.8 to 12.9 years, according to Bogin. The onset of puberty, however, is defined as the time when a girl's breasts start to develop. In the United States, it is 9.7 years for white girls, 8.8 years for black girls, 9.3 years for Hispanic girls and 9.7 years for Asian girls.

Studies have also pointed to a link between obesity and early puberty, as girls with higher body mass indexes (BMIs) are generally more likely to reach puberty at younger ages.

"The influence of BMI on the age of puberty is now greater than the impact of race and ethnicity," Dr. Frank Biro, a professor of pediatrics at Cincinnati Children's Hospital in Ohio, told Live Science in a 2013 interview.

And earlier puberty may have long-term health consequences, Biro said. For instance, studies have suggested that girls who mature earlier are more likely than those who mature later to develop high blood pressure and type 2 diabetes later in life.

There are also social consequences of earlier puberty; in some cultures, when a girl is biologically mature, she is also considered mature enough for marriage, Bogin noted. This may mean that she will not be able to continue her education or have a career once she does get married.

Therefore, the later a girl gets her first period, the better for her overall educational and life prospects. In fact, a Harvard study published in 2008 in the Journal of Political Economy showed that, in rural Bangladesh, where 70 percent of marriages occur within two years of menarche, each year that marriage is delayed corresponds to 0.22 additional year in school and 5.6 percent higher literacy.

Longevity and its bittersweet consequences

Humans are now living longer than ever, with average life expectancy across the globe shooting up from about 30 years old or so during the 20th century to about 70 years in 2012, according to the World Health Organization. The WHO predicts global life expectancy for women born in 2030 in places like the United States to soar to 85 years. The boost in life expectancy could be linked to significant advances in medicine, better sanitation and access to clean water, according to Bogin.

Although all of these factors have also greatly reduced mortality rates from infectious diseases, the deaths from degenerative diseases such as Alzheimer's, heart disease and cancer have been on the rise, Stearns said. In other words, people are living longer and are dying from different diseases than they did in the past.

"An American baby born in the year 2000 can expect to live 77 years and will most likely die from cardiovascular disease or cancer," Bogin said. [The Top 10 Leading Causes of Death]

As is often the case with biological advantages that humans sometimes gain, old age also comes with trade-offs.

"As more of us live longer, then more and more of us are encountering a death which is protracted and undignified," Stearns said. "So there are costs to all of this wonderful advance."

Autoimmune diseases such as multiples sclerosis and type I diabetes have also become more common, according to Stearns. Some scientists think the surge in such diseases is related to improved hygiene — the same factor that has allowed people to get rid of many infectious diseases, said Joel Weinstock, chief of gastroenterology at Tufts University Medical Center in Massachusetts. When the body is not exposed to any, or very few, germs, the immune system can overreact to even benign bugs, the thinking goes.

"Our theory is that when we moved to this super-hygiene environment, which only occurred in the last 50 to 100 years, this led to immune disregulation," Weinstock told Live Science in a 2009 interview. "We're not saying that sanitation is not a good thing — we don't want people to jog up to riverbanks and get indiscriminately contaminated. But we might want to better understand what factors in hygiene are healthy and what are probably detrimental, to establish a new balance and hopefully have the best of both worlds."

What is next for the human species?

It is hard so say what is in store for humans, as technology is changing the world so quickly.

"There is some fear out there that an esoteric cabal of scientists in white coats is going to take over the future of evolution with genetic engineering," Stearns said. "Whether we want to or not, we have already changed our future course of evolution, and it is not being done by some small group of people who are thinking carefully and planning, it is being done as a byproduct of thousands of daily decisions that are implemented with technology and culture."

"And we don't really know where that is going," he said, adding that, "once you accept that culture [including medicine, technology, media and transportation] has become a really strong driving force in human evolution, that is — we don't know how to predict culture."

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