Ascent of the Mammals


By Stephen Brusatte /Zhe-Xi Luo(羅哲西) S. ブルサット /Z.-X. ルオ
English 日本語 日本語
One early winter evening in 1824, English naturalist and theologian William Buckland rose to address the Geological Society of London. Anticipation filled the room. Buckland was known for his energetic lectures at the University of Oxford, where he would buzz around in full academic regalia, passing around severed animal parts and fossils to his adoring students. For years there had been rumors that Buckland had gotten his hands on some giant fossil bones, found by workers quarrying roofing stone in the English countryside. After nearly a decade of study he was finally ready to make an announcement. He told the audience that these bones had belonged to an ancient lizardlike animal much larger than any modern lizard. He called it Megalosaurus. The crowd was enraptured. Buckland had just unveiled the very first dinosaur.  1824年初冬のある晩,英国の博物学者で神学者のバックランド(William Buckland)は,ロンドン地質学会で講演した。会場は期待に満ちていた。バックランドはオックスフォード大学での熱のこもった講義で知られ,大学ではアカデミックガウンをまとい動物の体の一部や化石を彼を崇敬する学生たちに見せて回っていた。
That evening was a seminal moment in scientific history, touching off humankind’s enduring fascination with dinosaurs. But what is largely forgotten is that Buckland made another announcement that day, concerning a discovery much smaller in physical size but equally revolutionary. In reviewing the other fossils found alongside Megalosaurus in the stone quarries, Buckland had noted a “most remarkable” find: two tiny mammal jaws, about the size of mouse jaws. Until then, scholars thought mammals were a recent creation, appearing long after primeval geologic eras ruled by giant salamanders and lizards. The two minuscule jaws, their cusped teeth so unmistakably mammalian, were the first sign that this group had a much deeper history.  その晩は科学史上重大な一幕であり,以来,恐竜は人々を魅了してやまない存在となった。だが,ほとんど忘れられているが,バックランドはその日,もう1つ発表をした。サイズははるかに小さいが,同じくらい革新的な化石に関するものだ。バックランドは採石場でメガロサウルスと一緒に見つかった化石を調べ直していたとき,“最も驚くべき”ものに気づいた。それは2つの小さな哺乳類の顎で,ネズミの顎と同じくらいの大きさだった。
These jawbones raised a host of questions. How far back in the distant past did mammals originate? What were mammals doing during that long span of history when dinosaurs reigned? How did the classic mammal blueprint—with fur, mammary glands, big brains, complex teeth and keen senses, among other traits—originate? And why did one particular group of mammals—placentals like us (which give birth to live, well-developed young)—rise to dominance, with more than 5,000 species, running the gamut from featherweight bats to behemoth blue whales, spread across the planet today?  これらの顎の骨は多くの疑問をもたらした。哺乳類の起源はどれくらい古いのか? 恐竜が君臨していた長い間,哺乳類は何をしていたのか? 特に体毛や乳腺,巨大な脳,複雑な歯,そして鋭い感覚を持つ典型的哺乳類はどのようにして生まれたのか? 哺乳類のある特定グループ,すなわち私たち「有胎盤類」(よく成熟した子を産む)が優位になり,極めて軽いコウモリから巨大なシロナガスクジラまで5000を超える様々な種が生まれ,今日世界中に広がったのはなぜか。
For nearly two centuries after Buckland’s lecture these questions remained difficult to answer because so few fossils of early mammals were known. A surge of spectacular fossil discoveries over the past 15 years is at last allowing scientists to chart their evolutionary journey from the tiny critters living in the shadow of Megalosaurus to the astonishing array of forms that have come to rule the modern world.  初期哺乳類は化石がほとんどなかったため,バックランドの講義から2世紀近くこれらの疑問に答えるのは困難だった。しかしここ15年で素晴らしい化石が続々と発見され,メガロサウルスの陰で生きていた小動物から驚くほど多様な種が派生し,現代世界を支配するに至った哺乳類の進化の道筋が見えてきた。