原文網址：https://phys.org/news/2021-07-oldest-fossils-animals-canada.html
by Christina
Larson
根據星期三發表在期刊《自然》(Nature)的研究報告，一位加拿大的地質學家也許發現了地球上最早的動物化石紀錄。
這張沒有日期的照片由加拿大勞倫森大學的Elizabeth
Turner提供，顯示了她在加拿大西北領地的野外考察地點之一。根據2021年7月28日星期三發表在期刊《自然》的研究報告，這位加拿大的地質學家也許在這片區域發現了地球上最早的動物化石紀錄。圖片來源：Elizabeth
Turner
現今加拿大西北方一處遍布崇山峻嶺的區域，大約在十億年前是片史前海洋，該篇論文表示當時沉積下來的礦物裡面也許保存了原始海綿的遺骸。
地質學家Elizabeth
Turner在加拿大西北領地唯有靠直升機才能到達的偏遠區域發現了這些岩石，她從1980年代開始就在此地挖掘化石。從岩石薄片可以看到其中含有與現今的海綿骨骼類似的立體構造。
「我相信它們確實是古代的海綿，只有這類生物才擁有這種由絲狀有機物形成的網狀結構，」德國哥廷根大學的地質生物學家與海綿專家Joachim
Reitner,表示。他並未參與此研究。
鄰近岩層的定年結果指出這些樣品的年代大約為8.9億年，相較於之前發現最古老、確定為海綿的化石，它們的年代還要早了3.5億年。
「這項發現最令人驚訝的部分是年代，」瑞典烏普薩拉大學的海綿專家Paco
Cardenas表示。他也未參與此研究。「發現將進九億年前的海綿化石，勢必會大幅增進我們對於動物早期演化過程的認知。」
許多科學家相信最初的動物類型包括了柔軟的海綿或是類似海綿的動物，雖然它們缺乏肌肉與神經，但是已經擁有其他簡單的動物特徵，像是功能分異的細胞與精子。
可以肯定的是，任何年代為距今十億年前的事物都很難在科學界取得共識或肯定，因此其他研究人員未來可能會繼續審視並討論Turner的發現。
「我認為她的理由很有說服力，這確實十分值得發表――將證據呈現出來讓其他人可以加以檢視，」南加州大學的古生物學家David
Bottjer表示。他並未參與這項研究。
科學家相信地球的生命大約出現在37億年前，而最早的動物則要過了許久才出現，但是確切的時間仍有許多爭議。
目前為止，最早可以確定為海綿化石的年代大約是在5.4億年前，稱作寒武紀的時代。
但是科學家利用另外一條線索「分子時鐘」――分析兩物種的基因突變速率來回推牠們分家的時間――得出的現有證據指出海綿的出現時間要來得更早，大概是在十億年前。
然而，目前為止卻還沒有發現任何實體證據來支持。
「這是首次發現比寒武紀更加古老的海綿化石，而且不只是比較早，還早了非常多，這真的讓人非常興奮，」烏普薩拉大學的Cardenas說。他繼而表示研究似乎也確認了分子時鐘的估計結果。
動物在寒武紀才首次演化出堅硬的骨骼、外骨骼以及外殼這些比較容易保存下來的部位，因此在寒武紀之前的化石紀錄十分稀少。
「這類化石屬於比較複雜的動物――明顯的，在這之前勢必得有一段歷史屬於更加簡單的動物，像是最早出現的海綿，」論文作者Turner表示。她任職於加拿大安大略省的勞倫森大學。
距今8.9億年前這個時間非常重要，因為如果可以確定這些化石為海綿的話，就證實了最初的動物演化出來的時候，大氣和海洋的氧氣還沒有上升到科學家曾經以為要讓動物出現必須到達的濃度。雖然最近的研究已經顯示某些海綿可以在氧氣很少的情況下生存。
「地球上的所有生物都有個祖先。我們總是預測動物出現的第一道證據會相當微小而隱晦，不過是條十分細微的線索，」麻省理工學院的地質生物學家Roger
Summons表示。他並未參與此研究。
First sign of animal life on Earth may
be a sponge fossil
A Canadian geologist may have found the
earliest fossil record of animal life on Earth, according to a report published
Wednesday in the journal Nature.
Around a billion years ago, a region of northwest
Canada now defined by steep mountains was a prehistoric marine environment
where the remains of ancient sponges may be preserved in mineral sediment, the paper
says.
Geologist Elizabeth Turner discovered the rocks in a
remote region of the Northwest Territories accessible only by helicopter, where
she has been excavating since the 1980s. Thin sections of rock contain
three-dimensional structures that resemble modern sponge skeletons.
"I believe these are ancient sponges—only this
type of organism has this type of network of organic filaments," said
Joachim Reitner, a geobiologist and expert in sponges at Germany's University
of Gottingen, who was not involved in the research.
The dating of adjacent rock layers indicates the
samples are about 890 million years old, which would make them about 350
million years older than the oldest undisputed sponge fossils previously found.
"What's most stunning is the timing," said
Paco Cardenas, an expert on sponges at Sweden's Uppsala University, who was not
involved in the research. "To have discovered sponge fossils from close to
900 million years ago will greatly improve our understanding of early animal
evolution."
Many scientists believe the first animal groups
included soft sponges or sponge-like creatures that lack muscles and nerves but
have other features of simple animals, including cells with differentiated
functions and sperm.
To be sure, there's very little scientific consensus
or certainty about anything dating back a billion years ago, so other
researchers will likely continue to vet and debate Turner's findings.
"I think she's got a pretty strong case. I think
this is very worthy of publishing—it puts the evidence out there for other
people to consider," said David Bottjer, a paleobiologist at University of
Southern California, who was not involved in the research.
Scientists believe life on Earth emerged around 3.7
billion years ago. The earliest animals appeared much later, but exactly when
is still debated.
Until now, the oldest undisputed fossil sponges date
to around 540 million years ago, an era called the Cambrian period.
But scientists using a line of reasoning called the
molecular clock —where they analyze the rate of genetic mutations to backdate
when two species likely diverged—say that available evidence points to sponges
emerging much earlier, around a billion years ago.
Yet no supporting physical evidence has yet been
found until now.
"This would be the first time that a sponge
fossil has been found from before the Cambrian, and not only before, but way
before—that's what's most exciting," said Uppsala University's Cardenas,
adding that the research seems to confirm the molecular clock estimates.
Fossil evidence is scant before the Cambrian period
when animals first developed hard skeletons, exoskeletons and shells, which are
more likely to be preserved.
"Those kinds of fossils belong to more
complicated animals—obviously there has to be a back history" of simpler
animals like sponges emerging first, said the paper's author Turner, who is
based at Laurentian University in Ontario.
The dating of 890 million years ago is significant
because, if the sponge's identification is confirmed, it shows that the first
animals evolved before a time when oxygen in the atmosphere and ocean reached a
level scientists once thought was necessary for animal life. Yet recent
research shows that some sponges can survive with very little oxygen.
"Everything on Earth has an ancestor. It's
always been predicted that the first evidence of animal life would be small and
cryptic, a very subtle clue," said Roger Summons, an MIT geobiologist who
was not involved in the research.
原始論文：Elizabeth
C. Turner. Possible poriferan body
fossils in early Neoproterozoic microbial reefs. Nature, 2021. DOI:10.1038/s41586-021-03773-z
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