Why are polar bears white? Why do rabbits have long ears? Why do zebras have stripes (条纹)? It’s all about survival. Wild animals normally focus their efforts on two important tasks. One task is to look for things to eat. While they do that, they have to avoid getting eaten! They can’t afford to ignore any dangers around them. Life is just one long game of hide-and-seek. Luckily for them, the way they look usually helps them. The zebra is a very good example.

Zebras usually travel in large groups. Imagine a hundred zebras moving together across the grassland: in Africa. Not far away, a lioness is lying on a tree branch, resting. She is waiting for her chance to attack. The zebras are getting nearer, but they’ re running very close together. Now she’s seen them, and she’s getting ready: she knows she’ll soon have a zebra to kill.

The lioness’s goal is to catch a zebra for breakfast. There is no place to hide, but the zebras have a secret line of defence; their stripes.

Lions are colour-blind. As the zebras move in a large group, it’s hard for the lioness to see which zebra to attack. All she sees is a big mass of black and white stripes. She’s not sure what to do. Breakfast must wait. The animals are always trying to hide from her.

But do zebras’ stripes cause problems for other zebras as they do for lions? No, they don’t. Actually, the purpose of the stripes in this case is to help zebras recognize each other. Every zebra has an individual pattern of stripes and zoologists believe this is how zebras know who is who in the group. A mother zebra always recognizes her child among the crowd because its stripes are just a little different from the others.

The Nobel Prize in Physiology or Medicine was awarded to Sweden-born geneticist Svante Pääbo, recognizing him for discoveries related to the genomes (基因组) of extinct human groups that cast light on the genetic characteristics unique to present-day humans. The committee said Dr. Pääbo overcame challenges in recovering and analyzing ancient DNA to sequence (测序) the genome of the Neanderthal, an extinct relative of present-day humans. He also discovered a previously unknown human relative, called the Denisova.

Before Dr. Pääbo’s research, understanding of Neanderthals came from analysis of the features of ancient bone remains and from studies of tools and other archaeological artifacts (史前古器物). Dr. Pääbo’s work made it possible to precisely analyze the genetic characteristics of Neanderthals and helped illustrate how different types of humans mixed on the planet during periods of coexistence. Dr. Pääbo’s discoveries have laid the groundwork for research on how ancient gene sequences from extinct relatives influence the function of present-day humans. One example is a gene common among present-day Tibetans that makes those people better adapted to life at high altitude. It can be traced to the Denisovan genome.

Early in his career, Dr. Pääbo became focused on using advanced modern genetic techniques to study the DNA of Neanderthals. But DNA degrades over time meaning that, for ancient specimens (样本), only small amounts are left. When he worked in the University of Munich, he successfully sequenced DNA from a 40,000-year-old piece of bone. The DNA in question was so-called mitochondrial (线粒体的) DNA, a form of genetic material that is shorter, but more plentiful, than the extensive DNA found in the center of a cell. He went on to help establish the Max Planck Institute in Germany, where his research group continued to improve those methods with the aim of sequencing ever larger parts of an ancient genome. Those efforts ended in 2010.

Around the same time, the Max Planck group discovered that a 40,000-year-old finger bone found in the Denisova cave in the southern part of Siberia was unique when compared with all known sequences from Neanderthals and present-day humans. It marked the first time that a new type of human had been identified using only ancient DNA.

The first apes evolved 20 million years ago in the forests where they would go up a tree to get their food, and then come back down to move on to the next tree. Getting out of a tree presents all kinds of new challenges. Big apes can’t afford to fall because it could kill or badly injure them. Natural selection would have favored those with body structures that allowed them to descend safely.

Dartmouth researchers report in the journal Royal Society Open Science that apes probably evolved free-moving shoulders and flexible elbows to slow their descent from trees as gravity pulled on their heavier bodies. These flexible body parts passed on from ancestral apes would have allowed early humans to climb trees at night for safety and comedown in the daylight unhurt. When early humans transitioned from forests to open savannas (热带稀树草原), flexible shoulders and elbows were crucial for gathering food and using tools for hunting and defense.

The researchers used sports analysis and statistical software to study how primates(灵长目动物) like chimpanzees and small monkeys called mangabeys climbed in the wild. They found that chimps and mangabeys scaled(攀爬) trees similarly, with shoulders and elbows mostly bent close to the body. When descending, however, chimpanzees extended their arms above their heads to hold on to branches, much like a person using a ladder, to counteract(抵消) their heavier weight pulling them downward.

Luke Fannin, the lead author of the study, emphasized that their findings highlight the importance of “down climbing” in the evolution of apes and early humans. He explained, “Our research introduces the idea that down climbing played a significant role in shaping the structural differences between monkeys and apes, which eventually manifested in humans.”

“Our field has thought about apes climbing up trees for a long time, but we’ve been ignoring the second half of this behavior,” said Professor Jeremy DeSilva, “The findings are among the first to identify the significance of “down climbing” in the evolution of apes and early humans.