REMARKABLE CREATURES
神奇的生物
Translating Stories of Life Forms Etched in Stone
听石头讲述动物起源的故事
James G. Gehling
照片拍摄：詹姆斯 G 戈林（James G. Gehling）
PUZZLE Many of the creatures found in the fossil record from the time immediately preceding the Cambrian are so unlike modern forms that deciphering what they are and how they lived continues to challenge paleontologists.
谜团：很多化石形成的时间就在寒武纪之前，然而里面的生物却与现代生物迥异，解密这些生物是什么、习性如何，成了一直困扰古生物学家的一个难题。
By SEAN B. CARROLL
本文作者：肖恩 B 卡罗尔（Sean B. Carroll）
Published: July 26, 2010
发表于：2010年7月26日
In 1909, Charles Walcott, a paleontologist and secretary of the Smithsonian Institution, discovered one of the greatest and most famous fossil troves high in the Canadian Rockies on Burgess Pass in British Columbia. The slabs of Burgess Shale that Walcott excavated contained the earliest known examples at the time of many major animal groups in the fossil record, in rocks that were about 505 million years old.
1909年，史密森尼学会（Smithsonian Institution）秘书、古生物学家查尔斯·沃尔科特（Charles Walcott）在加拿大卑诗省内落基山脉上的伯吉斯垭口（Burgess Pass）发现了一个古化石群，是世界上最大、也是最著名的古化石群之一。沃尔科特在伯吉斯页岩中发现的古生物化石，囊括了五亿五百万年前地球上生活的多个主要物种，这是人类第一次发现这么古老的化石。
Walcott’s discovery was further evidence of the so-called Cambrian Explosion — the apparently abrupt appearance of complex animals in the fossil record within the Cambrian Period, from about 542 to 490 million years ago. Although not seen before on the scale documented in the Burgess Shale, the emergence of trilobites and other animals in the Cambrian was familiar to paleontologists, and had troubled Charles Darwin a great deal.
沃尔科特的发现也为所谓的“寒武纪大爆炸”提供了又一个证据。寒武纪大爆炸，指的是化石中所表现出的，在五亿四千两百万年前到四亿九千万年前的寒武纪时期，突然出现的许多高等生物。尽管以前没有发现过像伯吉斯页岩中这么大规模的三叶虫及其他寒武纪生物化石，它们也已经是古生物学家的老朋友、让达尔文大为头疼的老问题了。（译者注：寒武纪大爆发的事实曾让提出进化论的达尔文非常困惑，在《物种起源》中写道：“这件事情到现在为止都还没办法解释。所以，或许有些人刚好就可以用这个案例，来驳斥我提出的演化观点。”因为动物的大型化和多元化来得十分突然，进化的复杂性似乎被寒武纪蕴藏的神奇力量给简化了。直到今日，寒武纪大爆发仍是科学界的一大谜题，尚待更多的考古证据出土，也许就能窥见当时的实际情况，找出真正的原因。）
The difficulty posed by the Cambrian Explosion was that in Darwin’s day (and for many years after), no fossils were known in the enormous, older rock formations below those of the Cambrian. This was an extremely unsettling fact for his theory of evolution because complex animals should have been preceded in the fossil record by simpler forms.
在达尔文时代，以及之后的许多年，人们都没有在比寒武纪更古老的大型岩层中发现过化石，这对于进化论来说是个很不利的消息，因为按照进化论，高等生物之前应该有简单生物，这应该反映在化石上。
In “On the Origin of Species,” Darwin posited that “during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.” But he admitted candidly, “To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”
达尔文在《物种起源》中说：“在那个我们不了解的漫长的时期里，地球上住满了各种生物。”然而他也坦率地承认：“至于为什么像那么漫长的一段时期，我们却找不到它留下的一点点证据，我也无法给一个令人满意的回答。”
It took a very long time, and the searching of some of the most remote places on the planet — in the Australian Outback, the Namibian desert, the shores of Newfoundland and far northern Russia — but we now have fossil records from the time immediately preceding the Cambrian. The rocks reveal a world whose oceans were teeming with a variety of life forms, including primitive animals, which is certainly good news for Darwin.
科学家曾经花了很多时间，在地球上最偏僻的一些角落去寻找——澳大利亚内陆、纳米比亚沙漠、纽芬兰海岸、北部俄罗斯——现在好了，我们发现了寒武纪前的化石。化石告诉我们，那个时期的海底住着各种各样的生物，其中也包括一些原始生物，这对达尔文可是个好消息。
Now, this once-worrisome gap in the fossil record is a period of intense interest to geologists as well as paleontologists. The former have even given it its own division in the geological timescale. The Ediacaran Period, from 635 to 542 million years ago, is the first new geological period to be named in more than a century. Moreover, geologists have developed some intriguing theories about how dramatic changes in the Earth’s climate and chemistry during the Ediacaran may have allowed for the evolution of animals.
这个曾经令人头疼的化石空白期现在成为了地质学家和古生物学家共同的关注对象。地质学家甚至为它命名了一个新的地质时期——埃迪卡拉纪（the Ediacaran Period）。埃迪卡拉纪从六亿三千五百万年前持续到五亿四千二百万年前，是近一个多世纪以来科学家命名的第一个新的地质时期。地质学家还提出了一些很有趣的理论，来解释在埃迪卡拉纪时期剧烈的气候变化和化学成分变化下，地球上是如何进化出动物的。
The first major advance towards finding the earliest animal life occurred in 1946 when Reginald Sprigg, a geologist for the South Australia government, was checking out some old mines in the Ediacaran Hills of the Flinders Range several hundred miles north of Adelaide. Sprigg noticed some striking disc-shaped impressions up to four inches in diameter on the exposed surfaces of rocks nearby.
找寻最早的动物物种行动在1946年出现首次重大突破，供职于南澳大利亚政府的地质学家里基纳德·斯普里格（Reginald Sprigg），在澳大利亚阿德莱德以北几百公里的弗林德斯山脉（the Flinders Range）埃迪卡拉山一带考察一些古怪矿石的时候，在附近裸露的岩石上发现了一些圆盘状、最大直径达4英寸的印记，看上去十分惊人。
Sprigg interpreted the patterns as the fossil remains of soft-bodied creatures like jellyfish or their relatives. But when Sprigg first showed the imprints to leading authorities, they dismissed them as artifacts made by the weathering of the rocks. However, later that year, when Sprigg found the frond-like forms he called Dickinsonia , he was certain that such geometrical impressions could have been made only by living creatures.
斯普里格认为这是某种类似水母的软体动物化石。然而他第一次拿给权威专家看的时候，专家认为这只是岩石风化留下的痕迹，不值一提。同年晚些时候，斯普里格又发现了一些叶状的、他称之为“狄更逊水母（Dickinsonia）”的印记，他于是确信，这种地质印记只能是生物留下的。
Sprigg was excited by both the unusual appearance of the fossils and by their age, which he believed to be the beginning of the Cambrian, and made them the oldest animal forms yet seen. But despite their potential importance, Sprigg’s discoveries were ignored at an international geology meeting and his paper describing the fossils was rejected by the leading journal, . Sprigg moved on to other, more rewarding pursuits in the oil, gas, and mining industries.
斯普里格感到十分兴奋，因为化石外观很不寻常，并且他相信化石上的生物属于寒武纪起始的那段时期，这样一来，化石上的生物便是当时发现的最古老的动物物种了。尽管潜在的意义十分重大，斯普里格的这个发现却在一个国际会议上受到冷落，他描述这些化石的论文也被权威期刊退稿。斯普里格于是转而研究石油、天然气、矿产这些回报更高的课题去了。
Scientific attention to these strange forms was not revived until a decade later when more soft-bodied forms were found in the Ediacaran Hills and in England, and their age was firmly established as actually predating the Cambrian. Deposits of similar aged forms have been discovered at Mistaken Point on the Avalon Peninsula of Newfoundland, in southern Namibia, the White Sea of Russia, and more than 30 other locations on five continents. The global distribution of these disc-, frond-, tube-, branch-, or spindle-shaped forms demonstrate that life was complex and diverse in the Ediacaran.
直到10年后，人们在埃迪卡拉山地区和英国发现了更多此类软体动物的印记，科学家们才重新注意起这些奇怪的印记，并确定了这些印记的年代甚至要早于寒武纪。有类似古老印记的化石在纽芬兰阿瓦隆半岛的米斯特肯岬角（Mistaken Point），纳米比亚南部，俄罗斯白海，以及五大洲其他超过30个地方陆续被发现。各地发现的印记，有圆盘状，有叶状，有管状，有分枝状，有纺锤状，证明埃迪卡拉纪的生物形式不仅高级，而且多种多样。
But finding these fossils has posed many new mysteries. Many of the creatures are so unlike modern forms that deciphering what they are and how they lived continues to challenge paleontologists. Prof. Andrew Knoll of Harvard University has likened the Ediacaran forms to a paleontological “Rorschach” test because different scientists often interpret the same fossil very differently.
然而这些化石的发现又带来许多新的谜团。化石中有些生物与现代生物形式迥异，要搞清这些生物是什么、习性如何，对古生物学家仍是个挑战。哈佛大学的安德鲁·克诺尔（Andrew Knoll）教授将埃迪卡拉印记比作古生物学里面的罗夏墨迹测试（Rorschach test），因为不同的科学家对同一个化石的解读可能完全不同。
Dickinsonia, for example, has been interpreted as being a relative of jellyfish, a marine worm, a lichen, or even as a member of a completely extinct kingdom. The challenge to classifying most Ediacarans is that they lack some features that are characteristic of modern animals, a mouth or an anus in the case of Dickinsonia, or the shells and hard parts typical of many Cambrian groups. But, in fact, such simple bodies are exactly what should be expected of primitive forerunners of later animals.
例如，狄更逊水母，有的说是水母的近亲，有的说是海底蠕虫，有的说是地衣，有的甚至说它属于一个已经完全灭绝的物种。对埃迪卡拉动物群进行归类之所以困难，在于它们中的大多数都缺乏现代动物的某些典型特征，比如狄更逊水母就没有口，没有肛门，也没有大多数典型寒武纪动物所具有的外壳或硬甲。然而事实上，作为现代动物的前身，原始动物有这种简单的身体构造是十分自然的。
On the other hand, scientists have had to explain how such creatures functioned. Some of the very flat-bodied Ediacarans, for instance, lived on sediments and appear to have fed by directly absorbing nutrients by osmosis.
另一方面，科学家还需要弄清这些生物的生存机理。例如，有些埃迪卡拉动物身体非常扁，附着于沉积物上生活，似乎是靠渗透直接吸取养分而活。
The kinds of animals that paleontologists have been especially eager to identify in the Ediacaran are those with bilateral body symmetry, the feature characteristic of the majority of modern animal groups, including ourselves.
埃迪卡拉动物群中有一类是古生物学家特别想要认清的，就是两侧对称身体结构的，这是大多数现代动物包括人类的典型特征。
Bilateral animals flourished in the Cambrian so tracing their origins is crucial to understanding the pace of animal evolution. Several bilateral Ediacaran animals have been discovered, including Kimberella, a possible mollusc. Hundreds of Kimberella specimens are known that date to about 555 million years ago, 50 million years before the animals of the Burgess Shale.
两侧对称动物在寒武纪繁衍旺盛，因此追踪它们的起源对了解动物是如何进化而来的十分重要。科学家发现了多种两侧对称的埃迪卡拉动物，比如金伯拉虫（Kimberella）。金伯拉虫可能是一种软体动物，科学家已经知道金伯拉虫中有几百种，是来自五亿五千五百万年年前，这比伯吉斯页岩中的生物还要早五千万年。
The Ediacaran fossil record thus stretches the origins of animals to well before the Cambrian Explosion. But it also raises the question of why, after more than 2.5 billion years during which microscopic life dominated the planet, larger, more complex, forms emerged at that time?
埃迪卡拉化石的发现，将动物的形成时间从寒武纪又推早了不少。然而这一发现又提出了一个新的问题：在微生物统治地球25亿年之后，为什么在那个时候出现了体型更大、更高等的生物？
A key requirement for larger creatures is oxygen, and the dramatic history of oxygen levels is also etched in Ediacaran rocks. Geologists now understand that the earliest Ediacaran organisms were deep water creatures that emerged 575 to 565 million years ago, shortly after a major ice age ended about 580 million years ago.
体型更大的生物生存的关键因素是氧气，氧含量的变化史也刻在了埃迪卡拉岩石群上。地质学家现在了解到，最早的埃迪卡拉生物是在五亿七千五百万年至五亿六千五百万年前出现的深水生物，其出现恰好在五亿八千万年前一个主要的冰川季结束后不久。
Recent chemical analyses of Ediacaran sediments reveal that the deep ocean lacked oxygen before and during that ice age, then became much richer in oxygen and stayed that way after the glaciers melted . That sharp rise in oxygen may have been the catalyst to the evolution of animals, including our ancestors.
对埃迪卡拉沉积岩的最新化学分析表明，冰川季之前及其中，深海地区氧气缺乏，然而冰川溶化后，氧含量有了显著提高，并一直保持。氧含量的猛增可能是动物（包括人类祖先）出现的一剂催化剂。
Several weeks after the publication of “On the Origin of Species” and amid a torrent of criticism, Darwin added a mischievous postscript to a letter to his friend, the geologist Charles Lyell: “Our ancestor was an animal which breathed water, had a swim-bladder, a great swimming tail, an imperfect skull & undoubtedly was an hermaphrodite! Here is a pleasant genealogy for mankind.” The Ediacaran fossils tell us that Darwin was being too generous. Our earliest animal ancestor probably had no head, tail, or sexual organs, and lay immobile on the sea floor like a door mat.
《物种起源》出版几个星期后，在一片声讨声中，达尔文在给他的朋友，地质学家查尔斯·莱尔（Charles Lyell）写信时，恶作剧地写了一段附言，说：“我们的祖先是这样一种生物：在水中呼吸，长着鱼鳔，有一条大鱼尾，头骨不完全，而且毫无疑问，还是雌雄同体的！这就是人类的家系。”而埃迪卡拉化石告诉我们，达尔文太“慷慨”了。我们最早的动物祖先，可能既没有头，也没有尾，更没有性器官，连动都不能动，只能躺在海底，像一块门板。
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