原文網址：https://news.utexas.edu/2018/08/29/mammal-forerunner-sheds-light-on-brain-evolution
對腦部演化歷程的新瞭解：生育方法類似爬蟲類的哺乳類先驅
跟動物界的其他成員相比，哺乳類擁有最大的腦袋，產下的後代數目也是最少的其中之一。科學家最近描述了一具已經滅絕的哺乳類近親化石――還有它的38個嬰兒――得出了迄今最佳的證據顯示哺乳類演化過程中的關鍵之一是把生育能力拿去換取腦力。
此化石家庭是由德克薩斯大學奧斯汀分校的研究人員發現並進行相關研究。他們表示這項發現的絕無僅有之處是它含有已知唯一的哺乳類先驅的幼獸化石。不過同時出現這麼多具幼獸――比現存哺乳類平均一次生下的兩倍還多――顯示它的繁殖方式更接近爬蟲類。研究人員認為這些幼獸死亡的時候可能還在蛋裡發育，或者剛孵化不久而已。
有關這些樣本的描述8月29日發表在期刊《自然》(Nature )。研究人員表示它們或許有助於揭露哺乳類是如何演化出跟牠們祖先不同的繁殖方式，也就是不採用產生大量後代子孫的繁殖方式。
「這些幼獸處在演化樹上至關重要的位置。」領導這項化石研究的Eva Hoffman表示。當時她是德克薩斯大學傑克遜地球科學院的研究生。「牠們身上有許多和現代哺乳類相似的特徵，這在瞭解哺乳類的演化過程時有很深的關聯。」
研究共同作者為Hoffman的指導教授，傑克森研究院的Timothy Rowe。
這種已經滅絕的哺乳類動物近親跟恐龍同時生活在大約1.85億年前，是一種跟米格魯差不多大的草食動物，稱為Kayentatherium
wellesi(卡岩塔獸屬)。牠可能和哺乳類一樣具有毛髮。
研究人員發現的這些化石屬於Kayentatherium(卡岩塔獸屬)，一種生活在侏儸紀早期已經滅絕的哺乳類近親。圖片來源：Eva Hoffman / 德克薩斯大學奧斯汀分校
18多年前Rowe從亞利桑那州的岩層取得這具化石。當時他認為他帶回來的樣品只有一隻動物，卻不知道裡面還有數十隻幼獸。
2009年，在化石出土一年之後，之前為傑克森學院研究生和化石清修師的Sebastian Egberts在拆封這具化石的時候注意到一小塊牙釉質碎片，這是裡面有幼獸化石的第一筆跡象。
Egberts表示：「它看起來不像是魚類的尖牙或者是原始爬蟲類的小型牙齒。」他現在是費城骨科醫學院的解剖學講師。「它更像是臼狀齒，這讓我感到十分興奮。」
當時的電腦斷層掃描顯示岩石內部有幾塊骨頭。不過接下來的七年電腦斷層掃描成像技術有了大幅進展。在德克薩斯大學奧斯汀分校，高分辨率X光計算機斷層掃描所的技術人員提供的專業協助之下，Hoffman進行了大量數位影像處理顯示出內部還有其他幼獸化石――不只是顎骨和牙齒，還有完整的顱骨和一部份的身體骨骸。
Hoffman製作的3D顯影讓她可以對化石進行深入分析，結果證實這些微小的骨頭確實是幼獸，而且牠們和成體是同一物種。此外，分析顯示這些幼獸的顱骨就像是成體的縮小版――牠們的頭骨是成體的十分之一，而其他地方的比例也都相同。這項發現跟哺乳類相反，哺乳類的嬰兒出生時為了容納體積較大的腦部，因此面部會較短而頭顱則呈球狀。
腦部是相當耗能的器官，而懷孕的過程――更別說是育幼――也會消耗大量能量。雖然卡岩塔獸在其他方面跟哺乳類有很多共同點，但發現牠們的腦容量較小且一次會生產許多胎，代表在哺乳類的演化過程中相當重要的一步是把產下大量胎兒拿去換取更大的腦。這一步會在稍後的哺乳類演化中上演。
Rowe說：「在短短數百萬年之後的哺乳類無疑具有更大的大腦，而牠們產下的胎兒數目也更少。」
哺乳類的繁殖策略跟人類的出現有直接關聯，包括我們的大腦是如何形成。Rowe表示透過回頭研究我們的早期哺乳類祖先，人類可以更詳細地知道是什麼樣的演化作用塑造出我們這個物種。
他說：「現在我們擁有的研究技術可以讓我們從更多像這樣非比尋常的化石中挖掘出更為深入的故事，藉此來瞭解演化的進程，以及哺乳類的智能、行為和生理特徵是如何演化出來。」
研究經費來自於美國國家科學基金會、德克薩斯大學地質基金會和傑克森地質科學院。
Mammal forerunner that reproduced like a reptile sheds light on brain
evolution
Compared
with the rest of the animal kingdom, mammals have the biggest brains and
produce some of the smallest litters of offspring. A newly described fossil of
an extinct mammal relative — and her 38 babies — is among the best evidence
that a key development in the evolution of mammals was trading brood power for
brain power.
The find is among the
rarest of the rare because it contains the only known fossils of babies from
any mammal precursor, said researchers from The University of Texas at Austin
who discovered and studied the fossilized family. But the presence of so many
babies — more than twice the average litter size of any living mammal —
revealed that it reproduced in a manner akin to reptiles. Researchers think the
babies were probably developing inside eggs or had just recently hatched when
they died.
The study, published in the
journal Nature on
Aug. 29, describes specimens that researchers say may help reveal how mammals
evolved a different approach to reproduction than their ancestors, which
produced large numbers of offspring.
“These babies are from a
really important point in the evolutionary tree,” said Eva Hoffman, who led
research on the fossil as a graduate student at the UT Jackson School of
Geosciences. “They had a lot of features similar to modern mammals, features
that are relevant in understanding mammalian evolution.”
Hoffman co-authored the
study with her graduate adviser, Jackson School Professor Timothy Rowe.
The mammal relative
belonged to an extinct species of beagle-size plant-eaters called Kayentatherium wellesi that
lived alongside dinosaurs about 185 million years ago. Like mammals, Kayentatherium probably
had hair.
When Rowe collected the
fossil more than 18 years ago from a rock formation in Arizona, he thought that
he was bringing a single specimen back with him. He had no idea about the
dozens of babies it contained.
Sebastian Egberts, a former
graduate student and fossil preparator at the Jackson School, spotted the first
sign of the babies years later when a grain-sized speck of tooth enamel caught
his eye in 2009 as he was unpacking the fossil.
“It didn’t look like a
pointy fish tooth or a small tooth from a primitive reptile,” said Egberts, who
is now an instructor of anatomy at the Philadelphia College of Osteopathic
Medicine. “It looked more like a molariform tooth (molar-like tooth) — and that
got me very excited.”
A CT scan of the fossil
revealed a handful of bones inside the rock. However, it took advances in
CT-imaging technology during the next seven years, the expertise of
technicians at UT Austin’s High-Resolution X-ray Computed Tomography Facility
(UTCT), and extensive digital processing by Hoffman to reveal the rest of the
babies — not only jaws and teeth, but complete skulls and partial skeleton.
The 3D visualizations
Hoffman produced allowed her to conduct an in-depth analysis of the fossil that
verified that the tiny bones belonged to babies and were the same species as
the adult. Her analysis also revealed that the skulls of the babies were like
scaled-down replicas of the adult, with skulls a tenth the size but otherwise
proportional. This finding is in contrast to mammals, which have babies that
are born with shortened faces and bulbous heads to account for big brains.
The brain is an
energy-intensive organ, and pregnancy — not to mention childrearing — is an
energy-intensive process. The discovery that Kayentatherium had a tiny brain and
many babies, despite otherwise having much in common with mammals, suggests
that a critical step in the evolution of mammals was trading big litters for
big brains, and that this step happened later in mammalian evolution.
“Just a few million years
later, in mammals, they unquestionably had big brains, and they unquestionably
had a small litter size,” Rowe said.
The mammalian approach to
reproduction directly relates to human development — including the development
of our own brains. By looking back at our early mammalian ancestors, humans can
learn more about the evolutionary process that helped shape who we are as a
species, he said.
“There are additional deep
stories on the evolution of development, and the evolution of mammalian
intelligence and behavior and physiology that can be squeezed out of a
remarkable fossil like this now that we have the technology to study it,” Rowe
said.
Funding for the research
was provided by the National Science Foundation, The University of Texas
Geology Foundation and the Jackson School of Geosciences.
原始論文：Eva A.
Hoffman, Timothy B. Rowe. Jurassic stem-mammal perinates and
the origin of mammalian reproduction and growth. Nature,
2018; DOI: 10.1038/s41586-018-0441-3
引用自：University of Texas at Austin. "Mammal
forerunner that reproduced like a reptile sheds light on brain
evolution."
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