小化石顯示體型縮小如何替成功的演化奠下基礎
哺乳類在過去2億年來經歷了非比尋常的演化過程。今天(2018.9.17)發表在《自然》(Nature)的新研究顯示過程中的關鍵一步是把體型縮小。
現代哺乳類的起源可以追溯至2億多年前的恐龍時代。在接下來的1.5億年間某些恐龍演化成史上體型最大的陸生動物，但現代哺乳類的共同祖先卻採取了另一種截然不同的策略――把體型變得非常小。
由英國和美國的科學家組成的國際團隊利用最新的電腦技術進行分析，以觀察我們矮小的哺乳類祖先的骨骼發生了什麼變化。
現代哺乳類的其中一項特徵是牠們的下顎僅由一塊包含牙齒的骨頭組成。相較之下，其他脊椎動物的下顎都是由至少五塊骨頭拼接起來的複雜結構。從化石可以看出在演化的路途上哺乳類祖先的下顎越來越簡化，並在形成新關節的同時把其他骨頭移至中耳來幫助聽力。
此團隊的研究重點為一個歷時已久的謎題：哺乳類祖先為何在簡化並重建下顎的過程中，還是能正常進食並維持聽力？研究人員利用X光電腦斷層掃描幾具頭顱和下顎化石來製作數位模型，接著在不同電腦模型中進行模擬。
他們的結果指出哺乳類化石的矮小體型可以大幅減少牠們進食的時候顎骨承受的壓力，卻還能保有足夠的力道捕捉並咬碎牠們的獵物，像是昆蟲。
主要作者英國伯明罕大學的講師Stephan
Lautenschlager博士表示：「我們的結果對哺乳類顎骨2億多年來的演化歷程提供了一個新的解釋。對我們的哺乳類祖先來說，讓體型縮小似乎相當重要，這讓牠們進食的時候顎骨受到的壓力減少，使牠們可以同時重建牠們的顎骨。」
主持這項研究的英國布里斯托大學教授Emily
Rayfield補充：「哺乳類顎部關節的演化歷程已經困惑古生物學家超過50年。我們的研究運用電腦來解釋我們的哺乳類祖先如何把一部份下顎骨頭納入複雜的聽覺系統，同時還能維持顎部的功能正常運作。這項研究驗證了使哺乳類在動物王國中獨樹一格的理論，並且提出可能的機制。」
Tiny fossils reveal how shrinking was essential for successful
evolution
A new study published
today in Nature shows that getting
smaller was a key factor contributing to the exceptional evolution of mammals
over the last 200 million years.
The origin of modern mammals can be
traced back more than 200 million years to the age of dinosaurs. But while
dinosaurs evolved to become some of the largest land animals, for the following
150 million years, the ancestors of all modern mammals pursued an entirely
different strategy: getting very small.
An international team of scientists from
the United Kingdom and the US have used modern computer analysis to take a look
at what happened to the skeleton of our tiny mammal ancestors.
Modern mammals are unique in having a
lower jaw consisting of just a single bone that bears teeth. In contrast, all
other vertebrates possess complex lower jaws formed by at least five or more
bones joined together. In the course of evolution, fossils show that the lower
jaw of mammalian ancestors became simplified and a new jaw joint was formed,
while some of the other bones moved into the middle ear to aid in hearing.
The team’s research focussed on the
long-standing question of how it was possible to simplify and restructure the
lower jaw, while still being able feed and hear. Using X-ray computed
tomography (CT) scanning of several fossil skulls and lower jaws, the
researchers generated digital models which were subjected to different computer
simulations.
Their results showed that the small size
of the fossil mammals significantly reduced the stresses in the jaw bones when
feeding, while still being powerful enough to capture and bite through prey,
such as insects.
Dr Stephan Lautenschlager, lead author
and lecturer at the University of Birmingham, said: "Our results provide a
new explanation of how the mammalian jaw evolved over 200 million years ago.
Getting very small appears to have been crucial for our mammalian ancestors.
This allowed them to reduce the stresses in the jaw during feeding and made the
restructuring of the jaw bones possible.”
Professor Emily Rayfield from the
University of Bristol who lead the study added: “The evolution of the mammalian
jaw joint has perplexed palaeontologists for over 50 years. Using computational
methods we can offer explanations to how our mammalian ancestors were able to
maintain a working jaw while co-opting bones into a complex sound detection
system. Our research is about testing ideas of what makes mammals unique among the
animal kingdom, and how this may have come about.”
原始論文：Stephan Lautenschlager,
Pamela G. Gill, Zhe-Xi Luo, Michael J. Fagan, Emily J. Rayfield. The role
of miniaturization in the evolution of the mammalian jaw and middle ear. Nature,
2018; DOI: 10.1038/s41586-018-0521-4
引用自：University of
Birmingham. "Tiny fossils reveal how shrinking was essential for
successful evolution."
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