Have you ever wondered when dogs first became “man’s best friend” and the world’s favourite pet? If you have then you’re not alone. When and where dogs first began living side-by-side with humans are questions that have stirred hot debate among scientists. There are a few hard facts that all agree on. These include that dogs were once wolves and they were the first animal to be domesticated (驯养) by humans. They came to life some 15, 000 years ago, before the dawn of agriculture.

Beyond that, there is little agreement. The earliest bones found that are unquestionable dogs and not wolves date from 14,000 years ago. However, 30,000-year-old skulls (头骨) have been discovered in France and Belgium that are not pure wolves and that some scientists think could be dogs.

With such puzzling evidence, many scientists are now turning to DNA to find out when and where dogs were first domesticated. In one research project, tens of thousands of blood samples have been taken from street dogs around the world. The plan is to compare them with those of wolves. It’s even possible to analyse DNA from ancient bones. Tiny pieces of the 30,000-year-old skulls mentioned earlier are currently being studied, and another DNA study has already shown that ancient dogs preserved in the Alaskan ice fields evolved (进化) from Asian wolves, not American ones.

Indeed, the ancient DNA may turn out to be more informative than the DNA of living dogs. Because dogs have accompanied humans around the world for thousands of years, their current distribution may tell us very little of their origins. This is why different groups of scientists believe that dogs variously originated in eastern Asia, Mongolia, Siberia, Europe or Africa.

But why were the animals domesticated in the first place? The most recent theory is that dogs domesticated themselves, originally living in and around our ancient villages to eat any food thrown out. Today, this is a way of life still shared by three quarters of a billion unowned dogs worldwide.

We all think plants were expected to get larger with increased carbon dioxide in the atmosphere, but changes in temperature, humidity(湿度)and nutrient availability seem to havetrumpedthe benefits of increased carbon dioxide” said researchers from the National University of Singapore.

45 percent of the species studied now reach smaller adult sizes than they did in the past. The researchers pointed out that warmer temperatures and changing habitats, caused by climate change, are possible reasons for shrinking creatures.

“ We do not yet know the mechanisms(机制)involved, or why some organism are getting smaller while others are unaffected,” the researchers said. “Until we understand more, we could be risking negative consequences that we can’t yet quantify. ’’

The change is big in cold-blooded animals. Only two decades of warmer temperatures are enough to make retiles (爬行动物)smaller. An increase of only 1 degree centigrade caused nearly a 10 percent increase in metabolism(新陈代谢). Greater use of energy resulted in tiny tortoises and little lizards. Fish are smaller now too. Though overfishing has played a part in reducing numbers, experiments show that warmer temperatures also stop fish growing.

Warm-blooded animals aren’t immune(免除)from the size change caused by climate change. Many birds are now smaller. Soay sheep are thinner. Red deer are weaken And polar bears are smaller, compared with historical records.

This is not the first time this has happened in Earth’s history. 55 million years ago, a warming event similar to the current climate change caused bees, spiders and ants to shrink by 50 to 75 percent over several thousand years. That event happened over a longer time than the current climate change.

The speed of modern climate change could mean organisms may not respond or adapt quickly enough, especially those with long generation times climate change will be shown in the future.

Previous studies have suggested that early risers are more likely to be happy and healthy while night owls experience worse sleep as well as more depression and anxiety during the day.

Led by Jessica Rosenberg at Aachen University, researchers scanned the brains of 16 early risers, 23 night owls and 20 people with intermediate sleeping hours. They found that the brains of night owls had less “white matter”— which speeds up the transmission (传输) of nerve signals — in brain areas associated with depression.

As you know, after people fly in an airplane from one time zone to another, they often suffer from a confused and tired feeling called “jet lag” because their body clocks are out of sync(不同步的) with the new time zone. It usually takes about a week for their bodies to adjust to the new time. But night owls always have difficulty syncing their bodies to the right time due to their brain structure. “It’s like they suffer from permanent jet lag,” said Rosenberg.

The good news is that it is possible for night owls to turn themselves into morning people. According to the researchers, night owls should try to spend as much time in the sunlight as possible and reduce their exposure to artificial light at night to force their body clocks to shift to a more normal rhythm.