讓哺乳類成為哺乳類的關鍵是什麼？科學家表示：脊椎
研究骨頭化石得到的新結論顯示出脊椎的重要性
哺乳類在許多方面都與眾不同：我們是溫血動物，而且相較於我們的爬蟲類表親也更加敏捷。
顯示哺乳類脊椎演化過程的系統發生樹，由下到上為帆龍(Edaphosaurus)、三叉尖齒獸(Thrinaxodon)和現今的老鼠。圖片來源：哈佛大學比較動物學博物館
但是由美國國家科學基金會補助，哈佛大學的研究人員Stephanie Pierce和Katrina Jones主持的新研究指出我們還有一項特點：脊椎的組成。該篇敘述研究人員成果的論文發表於上周的《科學》(Science)期刊。
「脊椎基本上就像是一條串珠，每一顆珠子就是一塊骨頭，也就是脊椎骨。」哈佛大學古脊椎動物學的研究員Pierce表示。「多數四足動物，比方說蜥蜴，每一塊脊椎骨的外觀和功能都是相同的。」
「但哺乳類的脊椎骨不一樣。不同的段落或區域，像是脖子、胸腔和下背部的脊椎骨形狀有很大的差異。此外它們也可以獨立運作，使得哺乳類可以適應許多種不同的生活方式，比方說跑步、飛行、挖洞或者攀爬。」
現生哺乳類的脊椎骨擁有明顯的特化現象。科學家認為分區的根本依據歷時已久，可以一路追溯到最早的陸生動物。
哺乳類的身體構造藍圖是現存動物種類中最千變萬化的――至少科學家是這麼認為。然而，新研究在探討化石紀錄之後對上述傳統論點提出質疑。
「現在還活著的動物當中沒有一種紀錄了哺乳類從『蜥蜴般』的祖先演化而來的過程。」研究主要作者Jones表示。「為了瞭解這個過程，我們必須深入化石紀錄，觀察已經滅絕的哺乳類祖先：非哺乳類的合弓類(synapsid)。」
包括脊椎在內，若要了解專屬於哺乳類的特徵起源，關鍵就在這種哺乳類的遠古祖先身上。
但研究化石可不容易。Jones表示：「化石本身已經夠罕見了，而要找到已經滅絕的動物化石中完全保留總數達到25塊以上的脊椎骨更是難上加難。」
為了解決這個問題，研究人員搜索世界各地的博物館館藏，以研究生活在3.2億年前左右的動物中保存最完整的化石。
「研究人員探索遙遠的過去之後，發現哺乳類的演化過程中，早期發生在脊柱的變化是相當重要的開端。」補助此研究的美國國家基金會地球科學部門的計畫主任Dena Smith表示。「我們今日所知的各式各樣哺乳類可以形成，都得歸功於脊椎隨時間經過產生的變化。」
Pierce、Jones和共同作者，美國芝加哥菲爾德博物館的Ken
Angielczyk研究了數十具脊椎化石，以及超過1000塊現生動物的脊椎骨，包括老鼠、鱷魚、蜥蜴和兩棲類。
他們想要找出哺乳類脊椎的分區是否跟過往認為的一樣起源得相當早，或者在哺乳類身上獨自出現了某種變化。
Pierce說：「如果理論是對的，脊椎的分區在演化過程中一路以來都沒有變化的話，那我們應該可以在非哺乳類的合弓類化石中看見跟現代哺乳類一樣的脊椎分區。」
實際情形並非如此。當研究人員比對脊椎骨的位置和形狀時有了驚人的發現：脊椎在哺乳類的演化過程中增加了新的區塊。
Jones說：「最早的非哺乳類合弓類的脊椎分區比現在的哺乳動物還少。」
大概在2.5億年前，非哺乳類合弓類在肩膀和前腳附近的脊椎演化出新的區域。此外，這些動物的前肢也開始出現了劇烈變化。
科學家相信這些變化同步出現可能是為了因應牠們走路和跑動方式的改變。
「在這些變化發生時形成脊椎骨和肩胛骨的組織可能需要進行某種溝通協調。」Pierce表示。「我們認為此交互作用造成我們祖先肩膀附近的脊椎形成新的區塊，同時也讓前肢演化出新的形狀和功能。」
接下來在骨盆附近出現另一個新的區塊，Pierce說：「在沒有長出肋骨的腰椎附近形成了最後一個區塊，在哺乳類適應不同環境時具有莫大幫助。」
建構哺乳類脊椎的最後一步可能跟Hox基因的變化有關，這個基因在個體發育早期脊椎進行分區時相當重要。
「我們成功把已經滅絕的動物骸骨中出現的變化，跟現代發育生物學和基因學中的概念連結起來。」Jones表示。「這些方法結合起來讓我們更加了解哺乳類之所以為哺乳類的原因。」
What makes a mammal a mammal? Our spine, say scientists
Study
of fossil bones leads to new conclusions about spine's importance
Mammals are unique in many ways. We're warm-blooded and agile in
comparison with our reptilian relatives.
But a new study, funded by
the National Science Foundation (NSF) and led by Harvard University researchers
Stephanie Pierce and Katrina Jones, suggests we're unique in one more way --
the makeup of our spines. The researchers describe their finding in a paper
published this week in the journal Science.
"The spine is
basically like a series of beads on a string, with each bead representing a
single bone -- a vertebra," said Pierce, curator of vertebrate
paleontology at Harvard. "In most four-legged animals, like lizards, the
vertebrae all look and function the same.
"But mammal backbones
are different. The different sections or regions of the spine -- like the neck,
thorax and lower back -- take on very different shapes. They function
separately and so can adapt to different ways of life, like running, flying,
digging and climbing."
While mammal backbones are
specialized, the regions that underlie them were believed to be ancient, dating
back to the earliest land animals.
Mammals made the most of
the existing anatomical blueprint, or so scientists believed. However, the new
study is challenging this idea by looking into the fossil record.
"There are no animals
alive today that record the transition from a 'lizard-like' ancestor to a
mammal," said Jones, lead author of the study. "To do that, we have
to dive into the fossil record and look at the extinct forerunners of mammals,
the non-mammalian synapsids."
These ancient ancestors
hold the key to understanding the origin of mammal-specific characteristics,
including the spine.
But studying fossils isn't
easy. "Fossils are scarce and finding extinct animals with all 25-plus
vertebrae in place is incredibly rare," Jones said.
To tackle this problem, the
researchers combed museum collections around the world to study the
best-preserved fossils of animals that lived some 320 million years ago.
"Looking into the
ancient past, an early change in mammals' spinal columns was an important first
step in their evolution," said Dena Smith, a program director in NSF's
Division of Earth Sciences, which funded the research. "Changes in the
spine over time allowed mammals to develop into the myriad species we know
today."
Pierce and Jones, along
with co-author Ken Angielczyk of the Field Museum in Chicago, examined dozens
of fossil spines, as well as more than 1,000 vertebrae of living animals,
including mice, alligators, lizards and amphibians.
They wanted to find out
whether mammal vertebral regions were as ancient as previously thought, or if
mammals were doing something unique.
"If vertebral regions
had remained unchanged through evolution, as hypothesized, we would expect to
see the same regions in the non-mammalian synapsids that we see in mammals
today," said Pierce.
But that doesn't seem to be
the case. When the researchers compared the positioning and shape of the
vertebrae, they found something surprising. The spine had gained new regions
during mammal evolution.
"The earliest non-mammalian
synapsids had fewer regions than living mammals," said Jones.
About 250 million years
ago, a new region evolved near the shoulders and front legs. Dramatic changes
also began to appear in the forelimbs of animals known as non-mammalian
therapsids.
These simultaneous
developments, the scientists believe, likely occurred in conjunction with
changes in how creatures walked and ran.
"There appears to be
some sort of cross-talk during development between the tissues that form the
vertebrae and the shoulder blade," Pierce said. "We think this
interaction resulted in the addition of a region near the shoulder as the forelimbs
of our ancestors evolved to take on new shapes and functions."
Later, a region emerged
near the pelvis. "It is this last region, the ribless lumbar region, that
appears to be able to adapt the most to different environments," said
Pierce.
The final step in building
the mammal backbone may be linked with changes in Hox genes, important to spine
regions early in their development.
"We've been able to
make connections among changes in the skeletons of extinct animals and ideas in
modern developmental biology and genetics," Jones said. "This
combined approach is helping us understand what makes a mammal a mammal."
原始論文：K. E. Jones, K. D. Angielczyk, P. D. Polly, J. J. Head, V.
Fernandez, J. K. Lungmus, S. Tulga, S. E. Pierce. Fossils reveal the complex
evolutionary history of the mammalian regionalized spine. Science, 2018; 361
(6408): 1249 DOI: 10.1126/science.aar3126
引用自：National Science Foundation. "What makes a
mammal a mammal? Our spine, say scientists."
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