科學家追溯人類牙齒的起源至最原始的有頷魚
人類牙齒的起源可以追溯至超過四億年前——當時長相怪異的盔甲魚首次發育出下頷並開始捕捉活生生的獵物，而當今的人類和另外六萬多種有頷脊椎動物都是這些魚類的後裔。最近由瑞典烏普薩拉大學領導的國際科學團隊和歐洲同步輻射裝置(ESRF)——具有歐洲最亮X光源的設施進行合作，首度以數位方法「解剖」了一百多年前在布拉格附近發現的具有牙齒的最原始有頷魚化石。這項發表在《科學》的成果令人驚訝的地方在於其顯示了牠們和現存生物的牙齒具有共同特徵，也讓我們對自身牙齒的起源有了全新觀點。
4億900萬年前現為捷克的一處熱帶珊瑚礁。最原始且具有牙齒的有頷脊椎動物之一Radotina從牠的藏身之處――一個巨大的鸚鵡螺空殼中冒出來獵捕食物。圖片來源： Jan Sovak。
現今有頷脊椎動物的牙齒顯現出某些共同的模式，像是新的牙齒通常是在舊的內側形成，然後再往外移動來取而代之(人類牙齒的生長模式有所不同，新的牙齒是長在舊的下方，位於顎骨的深處)。不過硬骨魚(也就是陸生動物的祖先)和鯊魚卻有一些差異，比方說鯊魚事實上完全沒有硬骨，牠們的骨骼是由軟骨組成。另外，鯊魚牙本質形成的鱗片和嘴裡的真牙也沒有和軟骨相連，而是鑲嵌在皮膚裡面；但在硬骨魚和陸生動物身上，牙齒卻總是和顎骨相連。除此之外，鯊魚要讓遭到磨損的牙齒脫落時只要把整顆牙齒從皮膚上掉下來就好；硬骨魚和陸生動物讓牙齒脫落時則要溶掉牙齒的基部。
這些歧異之處為牙齒的起源帶來了許多問題。目前為止，研究人員的研究重點放在生存於4億3千萬年到3億6千萬年前一群稱為節頸魚(arthrodire)的古魚類，牠們是已知唯一帶有牙齒的有頷脊椎動物支系。然而，研究人員卻難以理解牠們是怎麼演化出現生脊椎動物的牙齒，原因是節頸魚牙齒的位置和增長模式跟硬骨魚和鯊魚相比之下都有很大的不同。
掃描最原始的有頷魚
由烏普薩拉大學、捷克查理大學、英國倫敦自然史博物館、捷克布拉格國家博物館和ESRF組成的團隊，著手進行研究來判斷這種特殊的齒列是否真為我們牙齒的祖先，或者只是在演化成目前的有頷脊椎動物途中分岔出去的特化產物。
為了達成這項目的，他們轉而探討另一種稱為棘胸魚(acanthothoracid)的早期魚類。科學家認為牠們比節頸魚更加原始而且和十分早期的有頷脊椎動物關係密切。但研究棘胸魚的難處在於牠們的化石相當稀少且沒有一具是完整的，其中情況最好的為上個世紀初從捷克布拉格盆地中年代為四億出頭的岩石中採集而來的化石。由於這些骨骸無法和周圍的岩石分離，因此用平常的技術來研究牠們被認證為相當困難的事情，也因此從未受到仔細的研究。
研究人員利用ESRF的ID19光束線來進行同步加速微斷層攝影，使得他們可以在不破壞化石的情況下看到化石內部的立體構造。
烏普薩拉大學的Valéria
Vaškaninová是研究主要作者，她說：「結果真的相當驚人，比方說我們看到了保存情況良好的牙本質，這是完全沒人料想到的發現。」後續解析度更高的掃描結果讓研究人可以看到牙齒的生長模式，甚至是這些古代牙齒的牙本質內部完美保存下來的細胞空間。
棘胸魚如同節頸魚一樣，齒列都是和骨頭連在一起。代表硬骨魚和陸生動物在這方面都保有和祖先一樣的特色；而鯊魚的牙齒則是經過特化，僅和皮膚接在一起。這跟普遍認為鯊魚是活著的脊椎動物中較為原始的想法互相牴觸。另外棘胸魚的牙齒也和節頸魚一樣不會直接脫落。
與節頸魚的差異比預期的還大
不過在其他方面，棘胸魚的齒列和節頸魚卻有基礎層面上的差異。就像鯊魚、硬骨魚和陸生動物一樣，棘胸魚的新牙只會從內側開始生長，最老的牙齒就位在顎骨的邊緣附近。在這方面，棘胸魚的齒列出乎意料的現代化。
Vaškaninová解釋：「我們感到驚訝的地方是，這些牙齒完美符合預期中軟骨魚和硬骨魚的共同祖先應該有的齒列樣貌。」
這些帶有牙齒的骨骸在外側的皮膚上也帶有並非用來啃咬的小型牙本質構造，原始的硬骨魚也有相同的特徵，但節頸魚卻沒有。這點相當重要，因為這顯示棘胸魚的顎骨就位在口部的邊緣附近，而節頸魚的顎骨則位在口部更加裡側的地方。特別的是，一種棘胸魚(Kosoraspis)的小型牙本質構造的形狀看起來正過渡成附近的真牙；而另一種棘胸魚(Radotina)的真牙則和皮膚上的小型牙本質構造有著幾乎相同的形狀。這或許可以證明真牙是近代才從皮膚上的小型牙本質構造演化而成。
共同作者，烏普薩拉大學的教授Per
Ahlberg表示：「這些發現徹底改變了我們對牙齒起源的認知。」他接著說道：「雖然棘胸魚是有頷脊椎動物中最原始的種類，但牠們的牙齒在某些方面卻比節頸魚更加貼近現代的脊椎動物。而從牠們類似於硬骨魚的顎骨來看，牠們似乎是我們的直系祖先。因此，當你早晨對著浴室鏡子露齒而笑時，對你回以笑容的牙齒起源可以一路追溯至最早的有頷脊椎動物出現的時候。」
Scientists trace the origin of our
teeth from the most primitive jawed fish
The origin of our teeth goes back more
than 400 million years back in time, to the period when strange armoured fish
first developed jaws and began to catch live prey. We are the descendants of
these fish, as are all the other 60,000 living species of jawed vertebrates. An
international team of scientists led by Uppsala University, in collaboration
with the ESRF, the European Synchrotron, the brightest X-ray source, has
digitally 'dissected', for the first time, the most primitive jawed fish
fossils with teeth found near Prague more than 100 years ago. The results,
published in Science, show that their
teeth have surprisingly modern features and give a new insight into the origin
of our teeth.
Teeth in current jawed vertebrates reveal some
consistent patterns: for example, new teeth usually develop on the inner side
of the old ones and then move outwards to replace them (in humans this pattern
has been modified so that new teeth develop below the old ones, deep inside the
jawbone). There are, however, several differences between bony fish (and their
descendants the land animals) and sharks; for example the fact that sharks have
no bones at all, their skeleton is made of cartilage, and neither the dentine
scales nor the true teeth in the mouth attach to it; they simply sit in the
skin. In bony fish and land animals, the teeth are always attached to jaw
bones. In addition, whilst sharks shed their worn-out teeth entire, simply by
detaching them from the skin, bony fish and land animals shed theirs by
dissolving away the tooth bases.
This diversity raises many questions about the origin
of teeth. Until now, researchers have focused on fossils of a group of ancient
fish that lived about 430 to 360 million years ago, called the arthrodires,
which were the only stem jawed vertebrates in which teeth were known. However,
they struggled to understand how they could have evolved into the teeth of
modern vertebrates, as arthrodire teeth are so different in position and mode
of tooth addition in comparison to bony fish and sharks.
Scanning the most
primitive jawed fishes
A team from Uppsala University, Charles University (Czech
Republic), Natural History Museum in London (UK), National Museum in Prague
(Czech Republic) and the ESRF set out to determine whether this peculiar type
of dentition was really ancestral to ours, or just a specialised offshoot off
the lineage leading towards modern jawed vertebrates.
With this aim, they turned to the acanthothoracids,
another early fish group that are believed to be more primitive than the
arthrodires and closely related to the very first jawed vertebrates. The
problem with acanthothoracids is that their fossils are rare and always
incomplete. The very finest of them come from the Prague Basin in the Czech
Republic, from rocks that are just over 400 million years old, and were
collected at the turn of the last century. They have proved difficult to study
by conventional techniques because the bones cannot be freed from the enclosing
rock, and have therefore never been investigated in detail.
The researchers used synchrotron microtomography at
the ESRF’s ID19 beamline, which allowed them to visualise the internal
structure of the fossils in 3D without damaging them.
“The results
were truly remarkable, including well-preserved dentitions that nobody expected
to be there” says Valéria Vaškaninová, lead author of the study and scientist
from Uppsala University. Follow-up scans at higher resolution allowed the
researchers to visualize the growth pattern and even the perfectly preserved
cell spaces inside the dentine of these ancient teeth.
Like arthrodires, the acanthothoracid dentitions are
attached to bones. This indicates that bony fish and land animals retain the
ancestral condition in this regard, whereas sharks are specialized in having
teeth that are only attached to the skin – in contrast to the common perception
that sharks are primitive living vertebrates. Again, like arthrodires, the
teeth of acanthothoracids were not shed.
More different
from arthrodires than expected
In other ways, however, acanthothoracid dentitions
are fundamentally different from those of arthrodires. Like sharks, bony fish
and land animals, acanthothoracids only added new teeth on the inside; the
oldest teeth were located right at the jaw margin. In this respect, the
acanthothoracid dentitions look remarkably modern.
“To our surprise, the teeth perfectly matched our
expectations of a common ancestral dentition for cartilaginous and bony
vertebrates.” explains Vaškaninová.
The tooth-bearing bones also carry small non-biting
dentine elements of the skin on their outer surfaces, a character shared with
primitive bony fish but not with arthrodires. This is an important difference
because it shows that acanthothoracid jaw bones were located right at the edge
of the mouth, whereas arthrodire jaw bones lay further in. Uniquely, one
acanthothoracid (Kosoraspis) shows a gradual shape transition from these
dentine elements to the neighboring true teeth, while another (Radotina) has
true teeth almost identical to its skin dentine elements in shape. This may be
evidence that the true teeth had only recently evolved from dentine elements on
the skin.
“These findings change our whole understanding of the
origin of teeth” says co-author Per Ahlberg, professor at Uppsala University.
And he adds: “Even though acanthothoracids are among the most primitive of all
jawed vertebrates, their teeth are in some ways far more like modern ones than
arthrodire dentitions. Their jawbones resemble those of bony fish and seem to
be directly ancestral to our own. When you grin at the bathroom mirror in the
morning, the teeth that grin back at you can trace their origins right back to
the first jawed vertebrates.”
原始論文：Valéria Vaškaninová, Donglei Chen, Paul
Tafforeau, Zerina Johanson, Boris Ekrt, Henning Blom, Per Erik Ahlberg. Marginal dentition and multiple
dermal jawbones as the ancestral condition of jawed vertebrates. Science, 2020
DOI: 10.1126/science.aaz9431
引用至：European Synchrotron Radiation Facility.
"Scientists trace the origin of our teeth from the most primitive jawed
fish."
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