Fossils show evolutionary leap from the sea
Scientists have captured the moment that life crawled out of the water some 380m years ago with the discovery of three fossils in the Canadian Arctic.
The new species, called Tiktaalik after an Inuit word for fish, could become as potent an icon of evolution in action as Archaeopteryx, the link between dinosaurs and birds, scientists say.
"Previous fossils representing this evolutionary event have really been fish with a few land characteristics, or land vertebrates with a few residual fish characteristics," said Andrew Milner, of London's Natural History Museum. "These Canadian fossils show an animal that sits bang in the middle between the fish and land animals."
The team from the University of Chicago and the Academy of Natural Sciences in Philadelphia, whose findings are reported in the journal Nature today, battled high winds and low temperatures to pull the fossils from frozen rock.
"We did a few 'high fives' when we uncovered the [first] fossil but there is only so much celebrating you can do in the Arctic," said Neil Shubin, of the University of Chicago.
Ellesmere Island has long been known to be a good source of fossils from the Devonian era, when vertebrates first appeared on land, but it had not previously been explored because conditions there are inhospitable even during the summer collecting season.
Dr Shubin said he and his colleagues "were always looking over our shoulders for polar bears".
On the other hand, said Dr Milner, "the great thing about the Arctic is that there is very little vegetation and no topsoil, so the good fossils are easier to locate."
Although the skeletons were removed in 2004, it took more than a year's painstaking laboratory work to tease the bones from the rock - and reveal just how remarkable they were.
The longest skeleton is three metres long, with jaws 20cm across. Tiktaalik was a predator with sharp teeth, an alligator-like head and flattened body, living in what 380m years ago was a sub-tropical coastal plain with rivers meandering slowly to the sea, rather like the Mississippi delta today.
Tiktaalik was a mosaic. It had fish-like fins, scales and gills but its ribs, neck and head resembled those of a tetrapod (early land animal). Unlike any fish, Tiktaalik had a neck.
"The neck was one of the biggest surprises," said Ted Daeschler of the Philadelphia academy. "This freed the skull from the shoulder girdle and gave the animal extra mobility."
At the ends of Tiktaalik's fins the scientists found rudimentary wrists and finger-like bones.
"The skeleton of Tiktaalik indicates that it could support its body under the force of gravity, whether in very shallow water or on land," said Farish Jenkins of Harvard University, another member of the team. "This represents a very critical early phase in the evolution of all limbed animals, including us."
The research was funded by the US National Science Foundation and the National Geographic Society. The first casts of the fossils go on show to the public at the Science Museum in London today.
“鱼”爬上岸的第一步
科学家们在加拿大北极圈中发现的3具化石，让他们捕捉到3.8亿年前生命蹒跚走出海洋的那一幕。
这个新物种被命名为“提克泰里克”(Tiktaalik)，在爱斯基摩语中是“鱼”的意思。科学家们表示，就像恐龙与鸟类之间的中间环节“始祖鸟”(Archaeopteryx)一样，这一物种有可能成为进化过程的重要标志。
“先前能够代表这一进化事件的，实际上是带有少数陆地特征的鱼类化石，或是残留少量鱼类特征的陆地脊椎动物化石，”伦敦自然历史博物馆(London’s Natural History Museum)的安德鲁?米尔纳(Andrew Milner)表示。“在加拿大发现的这些化石，是一种恰好介于鱼类和陆地动物之间的动物。”
该研究组成员来自芝加哥大学(University of Chicago)和费城自然科学院(Academy of Natural Sciences in Philadelphia)，他们冒着大风和低温，把这些化石从冻结的岩石中挖掘出来。他们的发现刊登在最新一期《自然》(Nature)杂志上。
芝加哥大学的尼尔?舒宾(Neil Shubin)表示：“当发现（第一具）化石的时候，我们情不自禁地击掌相庆，在北极圈里，也只能这样庆祝了。”
很长时间以来，埃尔斯米尔岛(Ellesmere Island)以盛产泥盆纪化石而闻名。泥盆纪是脊椎动物最初在陆地上出现的时期。但科学家此前没有对该岛进行考察，因为当地条件极其恶劣，不适宜人类居住，即使在夏天收集季节也不例外。
舒宾博士表示，他和他的同事“经常要查看身后是否有北极熊”。
舒宾称，另一方面，“北极圈最大的优势在于植被极少，而且没有土层，所以比较容易找到上乘的化石。”
尽管这些骨骼化石在2004年就被挖掘出来，但科学家们在实验室里辛苦了1年多，才将这些骨头从岩石中分离出来――并揭示出它们是多么的非同寻常。
其中最长的一幅骨架足有3米，颌宽20厘米。提克泰里克是一种牙齿锋利的食肉动物，有着类似鳄鱼的头部和扁平的躯体，居住在距今3.8亿年前的一块亚热带滨海平原，平原上缓缓河流蜿蜒入海，颇像今天的密西西比三角洲。
提克泰里克仿佛是一种“镶嵌画”动物，有着像鱼一样的鳍、鳞和鳃，但其肋骨、颈部和头部却类似于四足动物(tetrapod，早期陆地动物)。与任何鱼类都不同的是，提克泰里克有脖子。
“它的脖子是最令人惊奇的地方之一，” 费城自然科学院的特德?德斯科勒(Ted Daeschler)表示。“这使它的头颅从肩胛带部位解脱出来，使它移动起来更加灵活。”
科学家们在提克泰里克鳍部的末段，还发现了雏形的腕关节及手指状骨骼。
“提克泰里克的骨骼显示，它能够在重力下支撑其身体，无论在极浅的水中还是陆地上，”研究小组成员、哈佛大学(Harvard University)的法里什?詹金斯(Farish Jenkins)表示，“这代表了包括人类在内有四肢的动物进化过程中非常关键的早期阶段。”
这项研究由美国国家科学基金会(US National Science Foundation)资助。这些化石的首批铸模将在伦敦科学博物馆(Science Museum)向公众展出。
