原文網址：https://phys.org/news/2021-08-early-evolved-freshwater-algae-fossils.html
根據一篇發表在期刊《科學》(Science)的最新研究報告，世人對於植物的想法或許得有所改變。研究人員重新檢視類似孢子的微體化石之後，發現它們的特徵質疑了我們對於陸生植物演化過程的固有理解。
在年代大約為4億8000萬年前的奧陶紀早期，發現了新的孢子化石組合。它們的特徵介於寒武紀有爭議的型態，以及奧陶紀晚期和志留紀沉積物中廣為認可的植物孢子之間。這道連結銜接了化石孢子與分子數據，而有助於解釋植物主軸的顯體化石為什麼要到7500萬年之後的志留紀才出現。圖片來源：Paul Strother
這些微體化石發現於六十幾年前從澳洲取得的岩石樣品，定年結果顯示它們的年代大約為4億8000萬年前的奧陶紀早期。之前根據演化的步調――也就是分子時鐘得到的見解，與科學家多年來蒐集到早期植物的實體證據――孢子化石之間有段時間差，而最新的定年結果將這段差距縮小了大約2500萬年。
波士頓學院的古植物學家Paul
Strother是新研究的共同作者之一，他說兩者之間更為一致支持了植物起源和一種淡水綠藻――「輪藻」有關的演化發生模型。這項假設植物隨時間如何演化的模型當中，並非敘述一個物種大幅躍進成另一個物種的過程，而是詳細說明了植物如何從簡單的細胞分裂演化出最原始的胚胎期。
「我們發現了一群化石，它們可以把年代較早、疑似孢子的微體化石跟年代較晚、確定是由陸生植物產生的孢子化石連結起來，」Strother表示。「如果我們把陸生植物的起源視為一個漫長的過程，其中包含了胚胎發育方式的演變，那麼這些化石便有助於把孢子化石的紀錄對上分子時鐘的年代。」
Strother接著表示化石紀錄保存的證據，直接呈現了植物調控發育程序的基因組整體來說如何演化。這道過程一開始先演化出植物的孢子，接著形成了植物組織與器官的雛型，最後誕生出從巨觀來看完整的植物――或許跟現今的苔蘚有些類似。
「如果我們把孢子視為陸生植物演化過程中一個重要的環節，那麼分子時鐘的年代與化石紀錄復原的結果之間就不再具有時間差，」Strother表示。「沒有這些差異，我們便能更加清楚地瞭解這段演化過程中的全新階段：如何從簡單的單細胞生物變成複雜的多細胞生物。」
陸生植物是生命從水裡登上陸地的過程中相當重要的進展。Strother說這項結果或許讓研究人員與大眾得重新思考關於它們起源的看法。
「我們需要放棄將陸生植物的起源視為發生在某個特定時間點的想法，取而代之地，我們要整合古生代橫跨數百萬年的化石紀錄以及基因組整體的演化發生模型――尤其是從寒武紀到泥盆紀這段期間，」Strother表示。「這需要把之前判釋為真菌而非植物，但是無法確定的化石重新嚴格解讀才行。」
Strother和共同作者，澳洲國立大學(ANU)的Clinton
Foster起初只是為了描述一群類似孢子的微體化石，它們採自年代大約為4億8000萬年前奧陶紀早期的沉積物。Strother表示這些沉積物填補了孢子化石紀錄當中大約2500萬年的空白片段，連結了年代較晚並廣為認可的植物孢子，以及年代較早而有所爭議的化石型態。
Strother與Foster檢視了1958年從西澳北部鑽取出來的岩芯當中，孢子化石的族群如何組成。他們在波士頓學院及澳洲國立大學地球科學院，利用標準的光學顯微鏡來研究這些微體化石。由於它們的細胞壁含有相當堅韌的有機化合物，使得它們的結構可以保存下來而形成岩石。
「我們利用從岩芯當中挑出的孢子化石，建立了植物剛從藻類祖先演化出來時的演化史，」Strother表示。「我們對於岩石樣品有獨立的年代控制，因此可以透過孢子種類隨時間的變化來研究它們的演化過程。」
分子生物學家也會研究過去的演化史，方法是利用現在的植物基因來估計植物的起源時間，這種稱為「分子時鐘」的技術根據物種的基因組突變累積的平均速率，測量物種在演化道路上分歧的時間。
雖然如此，Strother表示直接從化石得到的數據和分子時鐘的年代卻有很大的差異，最多可達數千萬年。此外，最老的孢子以及第一個真正完整的植物，兩者之間也有類似的時間差。
Strother表示這些差異導致有些科學家假設化石紀錄中缺少了最早的陸生植物。
Strother表示：「我們的研究把化石紀錄整合到植物從藻類祖先誕生的演化發生模型之中，試著解決這些問題當中的一部份。」
Early land plants
evolved from freshwater algae, fossils reveal
The world may need to start thinking
differently about plants, according to a new report in the journal Science by researchers who took a fresh
look at spore-like microfossils with characteristics that challenge our
conventional understanding about the evolution of land plants.
Found in rock samples retrieved in
Australia more than 60 years ago, the microfossils dating to the Lower
Ordovician Period, approximately 480 million years ago, fill an approximately
25-million-year gap in knowledge by the molecular clock—or pace of
evolution—with the fossil spore record—the physical evidence of early plant
life gathered by scientists over the years.
This reconciliation supports an
evolutionary-developmental model connecting plant origins to freshwater green
algae, or charophyte algae, said Boston College paleobotanist Paul Strother, a
co-author of the new report. The "evo-devo" model posits a more
nuanced understanding of plant evolution over time, from simple cell division
to initial embryonic stages, rather than large jumps from one species to
another.
"We found a mix of fossils linking
older, more problematic spore-like microfossils with younger spores that are
clearly derived from land plants," said Strother. "This helps to
bring the fossil spore record into alignment with molecular clock dates if we
consider the origin of land plants as a long-term process involving the evolution
of embryonic development."
The fossil record preserves direct
evidence of the evolutionary assembly of the plant regulatory and developmental
genome, Strother added. This process starts with the evolution of the plant
spore and leads to the origin of plant tissues, organs, and eventually
macroscopic, complete plants—perhaps somewhat akin to mosses living today.
"When we consider spores as an
important component of the evolution of land plants, there is no longer a gap
in the fossil record between molecular dating and fossil recovery,"
Strother said. Absent that gap, "we have a much clearer picture of a whole
new evolutionary step: from simple cellularity to complex
multicellularity."
As a result, researchers and the public
may need to re-think how they view the origin of terrestrial plants—that
pivotal advance of life from water to land, said Strother.
"We need to move away from thinking
of the origin of land plants as a singularity in time, and instead integrate
the fossil record into an evo-devo model of genome assembly across millions of
years during the Paleozoic Era—specifically between the Cambrian and Devonian
divisions within that era," Strother said. "This requires serious
re-interpretation of problematic fossils that have previously been interpreted
as fungi, not plants."
Strother and co-author Clinton Foster, of
the Australian National University, set out to simply describe an assemblage of
spore-like microfossils from a deposit dating to the Early Ordovician
age—approximately 480 million years ago. This material fills in a gap of
approximately 25 million years in the fossil spore record, linking
well-accepted younger plant spores to older more problematic forms, said Strother.
Strother and Foster examined populations
of fossil spores extracted from a rock core drilled in 1958 in northern Western
Australia. These microfossils are composed of highly resistant organic
compounds in their cell walls that can structurally survive burial and
lithification. They were studied at Boston College, and at the ANU's Research School
of Earth Sciences, with standard optical light microscopy.
"We use fossil spores extracted from
rock drill cores to construct an evolutionary history of plants going back in
time to the very origin of plants from their algal ancestors," said
Strother. "We have independent age control on these rock samples, so we study
evolution by looking at changes in the kinds of spores that occur over
time."
Molecular biologists also look at
evolutionary history through time by using genes from living plants to estimate
the timing of plant origins using "molecular clocks"—a measurement of
evolutionary divergence based on the average rate during which mutations
accumulate in a species' genome.
However, there are huge discrepancies, up
to tens of millions of years, between direct fossil data and molecular clock
dates, said Strother. In addition, there are similar time gaps between the
oldest spores and when actual whole plants first occur.
These gaps resulted in hypotheses about a
"missing fossil record" of the earliest land plants," said
Strother.
"Our work seeks to resolve some of
these questions by integrating the fossil spore record into an evolutionary
developmental model of plant origins from algal ancestors," Strother said.
原始論文：P.
K. Strother & C. Foster. A fossil
record of land plant origins from charophyte algae. Science 13, 2021.
DOI: 10.1126/science.abj2927
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