《中国日报荐译》人类越来越笨了？

虽然我们不愿意承认，但人类事实上可能正变得日益迟钝。据一项新研究显示，人类颅骨持续增长了两百万年后，在过去2万年中又开始逐渐萎缩，而且全世界所有人不分性别、不分人种都这样。

古时的大头——图为在法国发现的一个2万8千年前头骨的3D复制图，它比我们的头骨要大20%。

       据发表于《探索》杂志上的一篇报道，过去2万年中，人类男性大脑平均体积已从1599立方厘米缩小至1350立方厘米，减少的体积相当于一个网球那么大。女性大脑萎缩幅度与男性差不多。
       不少学者认为随着人类的进化，人类确实是逐渐变笨了。不过，也有一些人不赞成这样的看法。威斯康辛州立大学人类学家约翰?霍克斯博士认为，人类大脑体积逐渐萎缩并不一定意味着人类智力下降。他说，我们消耗的所有能量中，大脑耗能就占到近20%，因此，头越大，需要的能量就越多，发育的时间就越长。他发现，在两万年前至一万年前那段时间，人口急剧增长，于是出现一种罕见的有利变异，可能是这种变异导致大脑结构开始简化、神经化学开始转变，从而使大脑容量增加了。
       一些古生物学者也支持霍克斯博士的结论，认为我们的大脑体积是越来越小了，但大脑运作效率却越来越高了。
       关于大脑体积逐渐萎缩之谜，学者们提出了不少理论来解释。其中一个理论是，在旧石器时代晚期，人类要在寒冷的户外活动，为了生存，头必须大；第二种理论是，过去人类的食物，如兔肉、鹿肉、狐狸肉及马肉等，不易嚼碎，于是颅骨变大了。但随着食物越来越易嚼烂，我们的颅骨便停止了增长。还有些专家称，过去婴儿死亡率高，只有最健壮的才能生存下去，而最健壮的往往头盖骨大。因此，婴儿死亡率逐渐下降后，我们的颅骨大小也就相应缩小了。
       近日，密苏里州立大学的两位研究认知能力的科学家戴维?盖瑞与德鲁?贝雷进行了一项研究，探索在190万年前至一万年前这段时间内，人类在适应日渐复杂的社会环境过程中其颅骨大小是如何变化的。他俩发现，人口密度低时，就像在人类进化过程中主要时期，人类颅骨增大，但当某个区域的人口数量增加后，颅骨则缩小。于是得出这样一个结论：随着人类社会结构日趋复杂，人脑变小了，因为人们不需多聪明就可以维持生存了。
       盖瑞博士认为我们的祖先在智力或创造力方面与我们相差甚远。他说：“经济专门化发展到一定程度后，农业发展、现代城市崛起，那些最聪明的人由此可以将精力集中于科学、艺术及其他领域”，而“祖先没有那样的基础条件来做这些事，他们所有的精力都用于生存，没有精力做其他事了。”
      不过，似乎人类的大脑又开始变大了。近日田纳西州立大学的人类学家理查德?简兹在测量了殖民晚期至二十世纪期间的非裔和欧洲裔美国人颅骨尺寸并进行比较后，发现我们人类大脑又开始增大了。

Are we becoming more stupid? Human brain has been 'shrinking for the last 20,000 years'

By Daily Mail Reporter
UPDATED: 18:05 GMT, 31 December 2010

• It's not something we'd like to admit, but it seems the human race may actually be becoming increasingly dumb.

Man's brain has been gradually shrinking over the last 20,000 years, according to a new report.

This decrease in size follows two million years during which the human cranium steadily grew in size, and it's happened all over the world, to both sexes and every race.

Old big head: A 3D image replica of a 28,000-year-old skull found in France shows it was 20 per cent larger than ours

'Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimetres to 1,350 cubic centimetres, losing a chunk the size of a tennis ball,' Kathleen McAuliffe writes in Discover magazine.

'The female brain has shrunk by about the same proportion.'

She was reporting on comments made by Dr John Hawks, an anthropologist from the University of Wisconsin, who argues that the fact the size of the human brain is decreasing doesn't necessarily mean our intelligence is in decline as well.

Some paleontologists agree with this diagnosis, that our brains may have become smaller in size, but increasingly efficient.

But others believe that man has indeed become steadily more stupid as he has evolved.

 

More...

• Did first humans come out of Middle East and not Africa? Israeli discovery forces scientists to re-examine evolution of modern man

Several theories have been advanced to explain the mystery of the shrinking brain. One is that big heads were necessary to survive Upper Paleolithic life, which involved cold, outdoor activities.

A second theory is that skulls developed to cope with a chewy diet of rabbits, reindeer, foxes and horses.

As our food has become easier to eat, so our heads have stopped growing, according to supporters of this theory.

Other experts say that with high infant mortality, only the toughest survived - and the toughest tended to have big heads. Therefore a gradually decreasing infant mortality rate has led to a proportionate decrease in the size of our brains.

As increasingly complex societies emerged, the brain grew smaller because people didn't have to be as smart to stay alive

A recent study conducted by David Geary and Drew Bailey, cognitive scientists at the University of Missouri, explored how cranial size changed as humans adapted to an increasingly complex social environment between 1.9million and 10,000 years ago.

They found that when population density was low, such as during the majority of our evolution, the cranium increased in size. But when a certain area's population changed from sparse to dense, our cranium size decreased.

They concluded that as increasingly complex societies emerged, the brain grew smaller because people didn't have to be as smart to stay alive.

But Dr Geary warns against stereotyping our ancestors as being more intelligent than us.

He said: 'Practically speaking, our ancestors were not our intellectual or creative equals because they lacked the same kind of cultural support.

'The rise of agriculture and modern cities based on economic specialisation has allowed the very brightest people to focus their efforts on the sciences, the arts and other fields.

'Their ancient counterparts didn't have that infrastructure to support them. It took all their efforts just to get through life.'

Dr Hawks, on the other hand, believes that the decrease in the size of our brains may actually show we are getting more intelligent.

The brain, he says, uses up to 20 per cent of all the fuel we consume. Therefore a bigger brain will require more energy and take longer to develop.

Dr Hawks notes that a boom in the human population between 20,000 and 10,000 years ago led to an unusual advantageous mutation to take place.

He believes this could have resulted in the brain becoming more streamlined, our neurochemistry shifting to boost the capacity of our brains.

But it seems the size of our brains could be on the increase again.

A recent study by anthropologist Richard Jantz of the University of Tennessee found that our brain size is on the increase again.

He measured and compared the craniums of Americans of African and European descent from late colonial times to the 20th century and found that our brain size is on the move again.