挑戰動物起源理論 丹麥海綿需氧極少
One of science's strongest dogmas is that complex life on Earth could only evolve when oxygen levels in the atmosphere rose to close to modern levels. But now studies of a small sea sponge fished out of a Danish fjord shows that complex life does not need high levels of oxygen in order to live and grow.
The origin of complex life is one of science's greatest mysteries. How could the first small primitive cells evolve into the diversity of advanced life forms that exists on Earth today? The explanation in all textbooks is: Oxygen. Complex life evolved because the atmospheric levels of oxygen began to rise app. 630 -- 635 million years ago.
However new studies of a common sea sponge from Kerteminde Fjord in Denmark shows that this explanation needs to be reconsidered. The sponge studies show that animals can live and grow even with very limited oxygen supplies.
In fact animals can live and grow when the atmosphere contains only 0.5 per cent of the oxygen levels in today's atmosphere.
"Our studies suggest that the origin of animals was not prevented by low oxygen levels," says Daniel Mills, PhD at the Nordic Center for Earth Evolution at the University of Southern Denmark.
Together with Lewis M. Ward from the California Institute of Technology he is the lead author of a research paper about the work in the journal PNAS.
A little over half a billion years ago, the first forms of complex life -- animals -- evolved on Earth. Billions of years before that life had only consisted of simple single-celled life forms. The emergence of animals coincided with a significant rise in atmospheric oxygen, and therefore it seemed obvious to link the two events and conclude that the increased oxygen levels had led to the evolution of animals.
"But nobody has ever tested how much oxygen animals need -- at least not to my knowledge. Therefore we decided to find out," says Daniel Mills.
The living animals that most closely resemble the first animals on Earth are sea sponges. The species Halichondria panicea lives only a few meters from the University of Southern Denmark's Marine Biological Research Centre in Kerteminde, and it was here that Daniel Mills fished out individuals for his research.
"When we placed the sponges in our lab, they continued to breathe and grow even when the oxygen levels reached 0.5 per cent of present day atmospheric levels," says Daniel Mills.
This is lower than the oxygen levels we thought were necessary for animal life.
The big question now is: If low oxygen levels did not prevent animals from evolving -- then what did? Why did life consist of only primitive single-celled bacteria and amoebae for billions of years before everything suddenly exploded and complex life arose?
"There must have been other ecological and evolutionary mechanisms at play. Maybe life remained microbial for so long because it took a while to develop the biological machinery required to construct an animal. Perhaps the ancient Earth lacked animals because complex, many-celled bodies are simply hard to evolve," says Daniel Mills.
His colleagues from the Nordic Center for Earth Evolution have previously shown that oxygen levels have actually risen dramatically at least one time before complex life evolved. Although plenty of oxygen thus became available it did not lead to the development of complex life.據外媒2月17日報道，科學界一直認定，地球上的複雜生物是在大氣中的氧氣含量達到現在的標準時才逐漸進化出來的。但是，學者研究丹麥海綿後認定，複雜生物的生存和發展並不需要高濃度的氧氣，並向傳統理論發起挑戰。
複雜生物的起源一直都是科學界的難解之謎。原始的細胞如何進化成了當今在地球上的高等生命形態？教科書上一般採用“氧氣”作爲答案，並解釋說6.3億至6.35億年前大氣中氧氣濃度上升，地球上因此進化出複雜的生命形態。
但是，南丹麥大學的博士生丹尼爾•米爾斯從丹麥一處峽谷中打撈出海綿，並加以研究後表示，需要重新審視這一傳統學說。研究發現，這些海綿在僅有當今大氣含氧量0.5%的極端環境下，仍能生存和成長。
米爾斯說：“我們的研究表明氧氣濃度偏低並不是阻礙生物起源的真正原因。”米爾斯和加州理工學院的劉易斯•M•沃德就此聯合撰寫論文，並且在《美國國家科學院院刊》上發表。
5億多年以前，“動物”這一複雜的生命形式剛剛出現。在此之前的數十億年裏，僅有的生命形式都爲單細胞生物。而就在大氣中氧氣含量顯著上升的時候，動物也恰巧出現，所以人們自然把這兩件事關聯起來，生物進化得益於氧氣增加。
米爾斯說：“但是據我所知，從來沒有人測試過動物究竟需要多少氧氣。所以我們決定一探究竟。”當今地球上海綿最接近早期生物，南丹麥大學附近就有。米爾斯近水樓臺，釣上幾個進行研究。米爾斯說：“我們把海綿放進實驗室，在氧氣含量僅爲當今大氣的0.5%時，它們仍能繼續呼吸和生長。”
實驗證明極低的含氧量沒有阻止生物的生長。米爾斯說： “肯定有其他的生態或進化機制發揮作用。也許需要一段時間才能產生出構建動物的生物機制，生命因此長期停留在單細胞狀態。也許進化出複雜、多細胞的生物就是很困難，所以地球上當時纔沒有動物。”
另有研究表明，在進化出複雜動物之前，地球上的氧氣含量已經顯著增加了至少一次。不過，豐富的氧氣並沒有直接帶來生物的進化。
