Image: Bonnie Squires looks at evolution exhibit
Joseph Kaczmarek  /  AP file
Bonnie Squires looks as casts of a human skulls on display at a University of Pennsylvania museum exhibit that tells the story of how the human race adapted, thrived and continues to evolve.
By
updated 6/16/2011 5:02:23 PM ET 2011-06-16T21:02:23

Humans might be evolving slower than scientists had thought, according to a new analysis of the genomes of two families, but there is a huge variance between people.

The researchers, reporting their findings June 12 in the journal Nature Genetics, based their measurement of evolution speed on the number of new mutations that occur during one generation in each of the families. A slower mutation rate means we probably separated from chimpanzees evolutionarily longer ago than previously thought, the researchers say, adding that the finding may have medical implications, if some groups of people are more mutation-prone than others.

"This makes us think about what are the underlying mechanisms of these mutations, other than just a random process," said study researcher Philip Awadalla, of the University of Montreal in Canada. "Why are there differences in the rate or accumulation of mutations in individuals?"

The mutations rate seems to be highly variable, Awadalla said, and could be affected by aging and environmental exposure to toxins, among other factors.

Tiny tweaks
Every person has two copies of each of their genes, one from the mother's egg and the other from the father's sperm. These two copies are quite similar, but have some random differences.

Then, when the person creates sperm (or eggs, if a woman), new changes occur by accident in the resulting copy of the genome that gets passed down to the offspring. By sequencing both the parents' and the child's genome for two families, they could see which mutations were already present and which were new to the offspring.

The researchers found that on average, humans seem to have about 60 new mutations passed down every generation — that's 60 changes out of 6 billion letters, or bases, that make up the genome. Previous methods, which indirectly calculated the rates, overestimated that number to be about 100 to 200, the researchers said.

This means that we are accumulating new genetic mutations — the foundation of evolution — about a third as quickly as previously thought. If this mutation rate has been steady throughout human evolution, it pushes the fork between humans and chimps back 7 million years earlier. Some earlier evidence indicates that chimps may be evolving quicker than humans, though Awadalla said they would like to see how they stack up using this whole-genome sequencing method.

Mutating mothers
Luckily in the case of the families the researchers studied, the mutations weren't dangerous. Most of them were outside of the genes, in areas of the genome called "non-coding," which don't carry protein-making codes to carry out any function.

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Scientists had hypothesized that because men make so many more copies of their genome when they make sperm (hundreds of millions every day), they might be more prone to pass on new mutations. The researchers found that this wasn't completely true. While one of the two families studied had 92 percent of their mutations passed down from the father, the other had only 36 percent.

"We always thought most of the mutations were coming from men," Awadalla told LiveScience. "In one family, there were more mutations in the male, but the next family the male had fewer mutations than the female did — it was very surprising."

The sources of these mutations seem to be diverse. They could be caused by age or other natural factors, since genome copying can get sloppy over time. It could also be due to genetic differences that control the DNA-copying machinery, which could affect the rate at which these mutations occur in different people. It's also possible that exposure to toxins in the environment could impact the mutation rate, Awadalla said.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

© 2012 LiveScience.com. All rights reserved.

Interactive: Before and after humans

Explainer: Ten fossils that tell the human tale

  • Frank Franklin II  /  AP file

    Where did we come from? Many truth seekers turn to faith and religion and find their answers therein. Others approach the question through a scientific lens and the theory of evolution. They have pieced together a tale of human origins from the fossils of our ancestors. The tale is incomplete and its telling reshaped with fresh interpretation of the growing fossil record.

    Click on the "Next" label to learn about ten fossil discoveries that have evolved the scientific rendering of human origins. In this image, a reconstructed Neanderthal skeleton is compared to a modern human.

    — John Roach, msnbc.com contributor

  • Toumai: Earliest-known ancestor of modern humans?

    MPFT

    Jaw fragments, isolated teeth and a skull excavated from the Sahel desert of Chad dated to between 6 million and 7 million years old may re-cast the opening chapter in the story of human origins. The fossils, revealed in 2001 and shown in this reconstruction, put the split in the evolutionary tree that eventually led to chimps on one branch and humans on the other more than 1,500 miles northwest of east Africa's Rift Valley, the current epicenter of research into human ancestors. But some scientists are not yet convinced the creature, named Sahelanthropus tchadensis and nicknamed Toumai, walked upright, which many scientists consider a key characteristic that distinguishes hominids from non-human primates.

  • Thigh bone suggests earliest two-legged walker

    Courtesy John Gurche, Brian Richmond via Science

    Analysis of a thigh bone amongst a clutch of fossils discovered in Kenya in 2000 and dated to nearly 6 million years ago may provide the earliest definitive evidence of a human ancestor that walked on two legs. Several detailed analyses of the femur, or thigh bone, shown here, have revealed it was adapted for upright walking. The bone belongs to a species known as Orrorin tugenensis. Most recently, U.S. scientists concluded the strategy first exhibited by this species for walking upright persisted for 4 million years, the majority of evolutionary history.

  • Middle Awash discovery fills gap in evolution story

    Tim D. White / Brill Atlanta

    Teeth and bones of the hand, foot, and thigh, shown here, are among the fossils of a 4.2 million-year-old Australopithecus anamensis specimen found in Ethiopia's Middle Awash region that has allowed scientists to link together their most complete chain of human evolution to date. The discovery helped fill a gap in the story, showing a likely transition between an earlier human ancestor known as Ardipithecus ramidus to the more recent australopithecines. The Middle Awash has yielded eight species in the story spanning 6 million years.

  • Lucy, the world's most famous fossil

    Dave Einsel  /  Getty Images

    Lucy, a 3.2 million-year-old Australopithecus afarensis named after the Beatles song "Lucy in the Sky with Diamonds," is perhaps the world's most famous fossil. She was discovered in the Afar region of Ethiopia in 1974 and remains among the most complete skeletons of an erect-walking human ancestor ever found, with about 40 percent of her bones intact. Her discovery allowed scientists for the first time to determine that upright walking predated the big brains of modern humans. Lucy's brain case is about the size of a chimp. In this photo, visitors view the Lucy skeleton at a Houston museum.

  • Taung child hailed as 'missing link' in 1924

    Denis Farrell  /  AP

    The diminutive fossil skull of 3.5-year-old early human ancestor, known as Taung child, was hailed as the "missing link" between apes and humans when it was discovered in 1924. Known scientifically as Australopithecus africanus, the discovery of the 2 million-year-old child also provided the first evidence that early humans evolved in Africa, rather than Europe, as many scientists believed at the time. In this photo, a researcher holds a replica of the skull as he makes the case that an eagle killed the Taung child.

  • Turkana boy, most complete skeleton found

    Sayyid Azim  /  AP

    Turkana boy, a nearly complete 1.6 million-year-old fossil of what some scientists call Homo ergaster, an early African population of Homo erectus, is considered the most complete skeleton of a prehistoric human ever found. The boy, who was discovered in 1984 in Kenya's Turkana region, stood 5-foot-3, indicating that hominids had gotten considerably taller and lankier since the days of Lucy, 3.2 million years ago. Plans to unveil Turkana boy at the National Museum of Kenya, shown here, in 2007 caused a stir between creationists and scientists.

  • Fossil discovery splinters human family tree

    Image: Casa Rinconada
    National Museums Of Kenya / F. Spoor  /  National Museum of Kenya via AP

    Many cartoons of evolution show a humpbacked ape slowly, linearly, progressing to a tall and erect modern human. Scientists long ago concluded that was too simple of a view, preferring instead to use a branching, thorny and knotted tree to depict the process. A discovery announced in 2007 threw yet another splinter in the picture. Many scientists had believed Homo habilis gave rise to Homo erectus who gave rise to modern humans. But the new finding shows habilis and erectus lived side by side for half a million years, raising doubt that habilis is a direct human ancestor. The scientists also found that erectus exhibited large size variation within the species, as shown in this image comparing two erectus skulls.

  • Neanderthals' relationship to modern humans fuzzy

    Image: Chankillo
    Image courtesy of National Academy of Sciences, PNAS

    The 1856 discovery of a skull cap and partial skeleton from a cave in Germany's Neander valley was the first recognized fossil human form. But exactly how the species, named in 1864 as Homo neanderthalensis, is related to modern humans remains the subject of fierce academic debate. Neanderthals occupied Europe and Asia from about 200,000 years to 30,000 years ago, overlapping in places with modern humans. Recent genetic analyses suggest little, if any, interbreeding between the species. Skeletal evidence, however, suggests Neanderthals were not very different than their modern human cousins. Even their brains were comparable to, if not bigger, than ours, as depicted in this Neanderthal reconstruction. Other studies have shown that like modern humans, Neanderthals used tools, wore jewelry, hunted, and buried their dead.

  • Hobbit discovery stuns the world, stirs debate

    Richard Lewis  /  AP

    As modern humans spread around the world over the past 160,000 years or so, a hobbit-like ancestor was holed up on the Indonesian island of Flores until at least 12,000 years ago, scientists announced at a press briefing in 2004, shown here. The stunning find has been scrutinized ever since. Some scientists agree the fossils represent a new species, Homo floresiensis. Others suggest the fossils belong to a diminutive race of modern humans, perhaps afflicted by one of several diseases associated with dwarfing.

  • Oldest modern humans found in Ethiopia

    Courtesy of Michael Day  /  AP

    The two partial skulls shown here of modern humans, Homo sapiens, were unearthed in Ethiopia in 1967. At the time, they were given a preliminary date of 130,000 years old. A 2005 revision using more modern dating techniques found them to be about 195,000 years old, making them the oldest known fossils of modern humans. Genetic evidence suggests modern humans arose in Africa about 200,000 years ago and then spread around the world, though other scientists hypothesize modern humans arose in parallel in Africa, Europe, and Asia.

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