８億９０００万年前の海綿構造、地球最古の動物化石か　研究

【パリＡＦＰ＝時事】カナダ北西部で発見された化石化した構造体が、８億９０００万年前の海に生息していた海綿動物である可能性を示す研究論文が先月２８日、英科学誌ネイチャーに発表された。これまで知られている中で地球上最古の動物となる可能性がある。（写真は資料写真）
　動物は地球の海洋と大気中に酸素が大量に放出された後に初めて発生したとする説が長年支持されているが、今回の結果はこの説に異を唱えるものだ。
　海綿は原始的で単純な動物で、現生する海綿類から得られた遺伝学的証拠により、１０億～５億年前に出現した可能性が高いことが明らかになっている。だが「新原生代」として知られるこの時代からはこれまで、海綿動物の体の化石による証拠は得られていなかった。
　今回、カナダ・ローレンシャン大学ハークウェイル地球科学院のエリザベス・ターナー教授は、炭酸カルシウムを沈殿させるバクテリアの一種によって形成された８億９０００万年前の生物礁で、海綿の証拠を探す調査を実施した。
　ターナー教授は、方解石の結晶を内包した微小な管状構造体の網状組織を発見した。これは管状構造体が、生物礁と同時期に発生したことを示唆していた。この構造体は現生する海綿類の一部の体内に見られる繊維状の骨格に酷似していた。
　発見された構造体が海綿の標本であることが立証されれば、これまで最古とされている海綿化石よりも３億５０００万年古いことになる。【翻訳編集ＡＦＰＢＢＮｅｗｓ】

〔ＡＦＰ＝時事〕（2021/08/12-10:05）

Fossilised structures discovered in northwestern Canada may be from sponges that lived in oceans as long as 890 million years ago, making them the earliest known animal life on Earth, research showed on Wednesday.
The findings also challenge the long-held idea that animals did not arise on Earth until after a major infusion of oxygen into the atmosphere and oceans.
Sponges are simple animals with an ancient history. Genetic evidence gathered from modern sponges has shown they likely emerged between 1 billion and 500 million years ago.
But until now there has been no evidence of fossilised sponge bodies from this period, known as the early Neoproterozoic era.
Elizabeth Turner, a professor at Canada's Laurentian University's Harquail School of Earth Sciences, looked for evidence of sponges in 890-million-year-old reefs that were constructed by a type of bacteria that deposited calcium carbonate.
She found networks of tiny tube-shaped structures containing crystals of the mineral calcite -- suggesting they were contemporaneous to the reef -- that closely resemble the fibrous skeleton found within some modern sponges.
If her structures Turner identified end up being verified as sponge samples, they will outdate the current oldest known sponge fossils by 350 million years.
Although the implications of her possible discovery, published in the journal Nature, Turner said she was not getting carried away.
The earliest animals to emerge evolutionarily were probably sponge-like. This too is not surprising, given that sponges are the most basic animal in the tree of animal life, she told AFP.
The nature of the material is familiar from the bodies of much younger body fossils of sponges, Turner said.
She said the possible sponges were around one centimetre across, and would have been tiny and inconspicuous, living in shadowy nooks and crannies below the upper surfaces of the reefs
If the structures do turn out to be confirmed as sponge specimens, that means they would have lived roughly 90 million years before Earth's oxygen levels reached levels thought to be necessary to support animal life.
Turner said that if confirmed to be sponges, she believed that they lived before the Neoproterozoic oxygenation event, during which oxygen levels increased, subsequently leading to the emergence of animal life.
If I am correct in my interpretation of the material, the earliest animals appeared before that event and may have been tolerant of comparatively low oxygen levels, relative to modern conditions, she said.
It is possible that the earliest animals were tolerant of low oxygen -- some modern sponges are -- but that more complex animal types that require a higher oxygen level did not appear until after the Neoproterozoic oxygenation event, Turner added.