生物谷：一个国际科学家研究小组近日发现了年代超过50万年的活细菌，这是迄今为止发现的最为古老的活有机体。这一发现将有助于人们更好地理解细胞老化以及探讨火星上存在生命的可能性等。相关论文发表在最新一期的《美国国家科学院院刊》（PNAS）上。
科学家知道，随着时间的流逝，所有的细胞最终都会消亡。但是有些细胞能更好地延迟老化和死亡的过程，甚至有些有机体还具有再生能力，可以修复损伤的细胞。这些有机体及细胞一直是科学家感兴趣的研究对象。
在最新的研究中，丹麦哥本哈根大学的Eske Willerslev教授领导他的国际研究小组从加拿大西北部挖掘出的永久冻结带（permafrost）中发现了这种年代超过50万年的古老细菌，它是活的，包含具有活性的DNA。
随后，Willerslev和同事从这种细菌的活细胞中分离出了DNA，并与大容量的基因文库（gene-bank）中的DNA作了比较，从而准确确定了该种细菌DNA的进化位置。
Willerslev表示，此次研究将帮助人们更好地理解细胞分解、延迟老化以及达尔文进化论等问题，并可为有关火星生命的讨论提供一些借鉴。不过，要弄清这些细胞长寿的原因以及循环进化、新旧DNA混合时间等问题，还需要更深入的研究。（科学网 梅进/编译）
英文原文:
Source: University of Copenhagen
Date: August 28, 2007
World's Oldest Bacteria Found Living In Permafrost
Science Daily — A research team has for the first time ever discovered DNA from living bacteria that are more than half a million years old. Never before has traces of still living organisms that old been found.
The exceptional discovery can lead to a better understanding of the aging of cells and might even cast light on the question of life on Mars. The discovery was made by Professor Eske Willerslev from the University of Copenhagen and his international research team.
All cells decompose with time. But some cells are better than others to postpone the decomposing and thus delay aging and eventually death. And there are even organisms that are capable of regenerate and thereby repair damaged cells. These cells -- their DNA -- are very interesting to the understanding of the process of how cells break down and age.
The research team, which consists of experts in, among other things, DNA traces in sediments and organisms, have found ancient bacteria that still contains active and living DNA. So far, it is the oldest finding of organisms containing active DNA and thus life on this earth. The discovery was made after excavations of layers of permafrost in the north-western Canada, the north-eastern Siberia and Antarctica.
The project is about examining how bacteria can live after having been frozen down for millions of years. Other researchers has tried to uncover the life of the past and the following evolutionary development by focusing on cells that are in a state of dead-like lethargy. "We, on the other hand, have found a method that makes is possible to extract and isolate DNA traces from cells that are still active. It gives a more precise picture of the past life and the evolution towards the present because we are dealing with cells that still have a metabolistic function -- unlike "dead" cells where that function has ceased," says Eske Wilerslev.
Professor Eske Willerslev collecting soil samples in the permafrost in the Yukon, Canada. Researchers found half million year old bacteria in some of the samples collected. (Credit: Duane Froese)
After the fieldwork and the isolation of the DNA, the researchers compared the DNA to DNA from a worldwide gene-bank in the US to identify the ancient material. Much in the same way the police compares fingerprints from a crime, the researchers were able to place the DNA more precisely and to place it in a context.
There is a very long way, of course, from our basic research towards understanding why some cells can become that old. But it is interesting in this context to look at how cells break down and are restored and thus are kept over a very long period. The researchers' methods and results can be used to determine if there was ever life on Mars the way we perceive life on earth.
And then there is the grand perspective in relation to Darwin's evolution theory. It predicts that life never returns to the same genetic level. "But our findings allows us to post the question: are we dealing with a circular evolution where development, so to speak, bites its own tail if and when ancient DNA are mixed with new?" says Eske Willerslev.
The discovery is being published in the current issue of PNAS (Proceedings of The National Academy of Sciences).
Note: This story has been adapted from a news release issued by University of Copenhagen.
