溫血的起源也許比之前認為得還要早上許多

原文網址：www.sciencedaily.com/releases/2017/05/170518104131.htm

溫血的起源也許比之前認為得還要早上許多

研究顯示此特性發展出來的時間可能要再往前推2000萬年

陸生動物的溫血特性在演化史上發展出來的時間點，可能比之前認為得還要早上許多。這項研究由德國波昂大學進行，近日發表於期刊《法國科學院報告：古生物學篇》(Comptes Rendus Palevol)。

喜歡觀察蜥蜴的人的最好機會通常是在早上，在一天的這個時刻，時常可以發現蜥蜴正在進行日光浴。這是因為牠們需要外在的能量供應來讓牠們達到所需的活動溫度。然而，老鼠跟其他哺乳類則是利用不同方式來讓自己保持在舒適的最佳溫度：牠們靠著燃燒熱量，使得牠們甚至能在嚴酷的寒冬夜晚，依然讓自己保持溫暖。

哺乳類因此被稱為溫血動物。直到現在，科學家認為「體內暖爐」大概是2億7000萬年前，在四足陸上動物的身上發明出來。「然而，我們的結果顯示溫血特質的形成時間可能早了2000萬到3000萬年。」波昂大學Steinmann地質、礦物和古生物研究所的Martin Sander教授如此解釋。

骨頭溫度計

對於滅絕許久的動物來說，要量測牠們的體溫自然是不可能用溫度計就能達成。然而，溫血特質會在化石中留下顯著的痕跡。溫血不只意謂動物不需仰賴周遭的溫度來維持體溫，也讓牠們的生長速度更快。Sander解釋：「這會在骨頭的構造中彰顯出來。」

骨頭是由蛋白質纖維、膠原蛋白和一種稱作羥磷灰石(hydroxyapatite)的生物材料組成。膠原蛋白纖維的排列方式越規律，骨頭就越穩定，但同時生長速度也比較慢。因此哺乳類的骨頭擁有一種特殊構造，讓它們在快速生長的同時仍然保持穩定。Sander說：「我們稱其為纖維板層(fibrolamellar)。」

Sander和他的博士生Christen D. Shelton(現任職於開普敦大學)合作，檢測了一種滅絕許久的陸生動物的肱骨和大腿骨。此動物為哺乳類的祖先――生活在3億年前的蛇齒龍(Ophiacodon)。Sander總結他們的分析結果：「即便是在蛇齒龍身上，牠們的骨頭也以纖維板層骨的構造生長。代表這些動物很可能已經具有溫血特性。」

蛇齒龍的身長可以達到兩公尺，但除此之外都跟今日的蜥蜴十分類似――這有相當好的理由：哺乳類和爬蟲類有親緣關係，因此牠們擁有共同的祖先。在演化樹上，蛇齒龍的所在位置相當靠近兩者分道揚鑣的地方。

第一隻爬蟲類是溫血動物？

然而，現今存活的蜥蜴、烏龜和其他爬蟲類是冷血動物。直到現在，科學家推測這是代謝類型的原始形式，也就是哺乳類和爬蟲類的共有祖先是冷血動物。因此溫血特性是之後才在哺乳類的演化路途中出現。

但是蛇齒龍出現的時間點位在哺乳類和爬蟲類分歧之後不久。Sander表示：「這引發了一道問題：蛇齒龍的溫血特性實際上是全新演化出來的性質；或者是遠在分歧出現之前，最初一批陸生動物就是溫血動物？」雖然這只是猜測，然而如果該理論為真，我們就得徹底修正我們的想法：最初的爬蟲類其實同樣是溫血動物，但牠們之後卻揚棄了這種代謝方式。

Warm-bloodedness possibly much older than previously thought

Characteristic may have developed 20 million years earlier, study shows

Warm-bloodedness in land animals could have developed in evolution much earlier than previously thought. This is shown by a recent study at the University of Bonn, which has now been published in the journal Comptes Rendus Palevol.

People who like watching lizards often get the best opportunity to do so in the morning, as they can usually be found sunbathing at this time of day. This is because they rely on an external energy supply to reach their operating temperature. However, mice and other mammals make themselves nice and cozy in a different way: they burn calories and can even keep themselves warm during a bitterly cold winter's night.

Mammals are thus referred to as warm-blooded. Until now, it was thought that the "body heater" was invented in four-legged land animals around 270 million years ago. "However, our results indicate that warm-bloodedness could have been created 20 to 30 million years earlier," explains Prof. Martin Sander from the Steinmann Institute for Geology, Mineralogy and Paleontology at the University of Bonn.

Bones as a thermometer

For long-extinct animals, it is naturally not possible to simply determine body temperature using a thermometer. However, warm-bloodedness leaves behind tell-tale signs in fossils. It not only means that the animal is not reliant on the ambient temperature, but also enables faster growth. "And this is shown in the structure of the bones," explains Sander.

Bones are composites of protein fibers, collagen, and a biomaterial, hydroxyapatite. The more orderly the arrangement of the collagen fibers, the more stable the bone, but the more slowly it normally grows as well. The bones of mammals thus have a special structure. This allows them to grow quickly and yet remain stable. "We call this bone form fibrolamellar," says the paleontologist.

Together with his PhD student Christen D. Shelton (now at the University of Cape Town), the scientist looked at humerus bones and femurs from a long-extinct land animal: the mammal predecessor Ophiacodon. This lived 300 million years ago. "Even in Ophiacodon, the bones grew as fibrolamellar bones," says Sander to summarize the analysis results. "This indicates that the animal could already have been warm-blooded."

Ophiacodon was up to two meters long, but otherwise resembled today's lizards -- and not without good reason: mammals and reptiles are related; they thus share a predecessor. In the family tree, Ophiacodon is very close to the place where these two branches separate.

Were the first reptiles warm-blooded?

However, lizards, turtles and other reptiles living today are cold-blooded. Until now, it has been assumed that this was the original form of the metabolism -- i.e. that the shared ancestor of both animal groups was cold-blooded. Warm-bloodedness would thus be a further development, which arose over the course of mammalian evolution.

However, Ophiacodon appears a very short time after the division between mammals and reptiles. "This raises the question of whether its warm-bloodedness was actually a completely new development or whether even the very first land animals before the separation of both branches were warm-blooded," says Sander. That is just speculation. However, if this theory is correct, we would have to drastically correct our image: the first reptiles would then also have been warm-blooded -- and would have only discarded this type of metabolism later.

原始論文：Christen D. Shelton, Paul Martin Sander. Long bone histology of Ophiacodon reveals the geologically earliest occurrence of fibrolamellar bone in the mammalian stem lineage. Comptes Rendus Palevol, 2017; DOI: 10.1016/j.crpv.2017.02.002

引用自：University of Bonn. "Warm-bloodedness possibly much older than previously thought: Characteristic may have developed 20 million years earlier, study shows." ScienceDaily. ScienceDaily, 18 May 2017.