Earth’s earliest non-marine eukaryotes

• Published in: Nature (London) Vol. 473; no. 7348; pp. 505 - 509
• Main Authors: Strother, Paul K., Battison, Leila, Brasier, Martin D., Wellman, Charles H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 26-05-2011; Nature Publishing Group
• Subjects: 631/158/2459; 631/181/414; Aquatic Organisms; Biological Evolution; Biota; Clay minerals; Cyanobacteria; Discovery and exploration; Earth; Earth sciences; Earth, ocean, space; Ecosystem; Eukaryota - classification; Eukaryota - cytology; Eukaryota - isolation & purification; Eukaryotes; Exact sciences and technology; Exposed habitats; Fossils; Fresh Water; Geologic Sediments - analysis; Geologic Sediments - chemistry; History, Ancient; Humanities and Social Sciences; Invertebrate paleontology; letter; multidisciplinary; Natural history; Paleontology; Precambrian; Science; Science (multidisciplinary); Scotland; Sedimentary structures; United Kingdom; Scotland; Great Britain; Europe; Western Europe; United States; British Isles, Scotland; USA, Arizona; microfossils; habitat; bacteria; aquatic environment; nodules; Paleosols; asymmetry; Precambrian; preservation; sheet silicates; Proterozoic; silicates; soils; upper Proterozoic; clastic rocks; shale; biota; fresh-water environment; morphology; siliciclastics; Torridonian; microscope methods; chemically precipitated rocks; sedimentary rocks; subaerial environment; mineralization; prokaryotes; sedimentary structures; phosphate rocks; Cyanophyta; terrestrial environment; alteration; clay minerals
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09943

An early move to fresh water
Fossil discoveries in billion-year-old Precambrian shales from the Torridonian of the northwest Scottish Highlands suggest that the eukaryotes that evolved to live on the land emerged from the sea earlier than previously thought. Life originated in the sea more than three billion years ago, but the timing of the first stirrings on land is less clear. The microfossil assemblages were deposited in former lake beds and siliciclastic microbial mats, and represent members of a diverse population of multicellular organisms up to 1 millimetre long, with organic walls. They appear to have been simple eukaryotes that lived in freshwater, and were perhaps partly exposed to the atmosphere at times.
The existence of a terrestrial Precambrian (more than 542 Myr ago) biota has been largely inferred from indirect chemical and geological evidence associated with palaeosols
1
,
2
, the weathering of clay minerals
3
and microbially induced sedimentary structures in siliciclastic sediments
4
. Direct evidence of fossils within rocks of non-marine origin in the Precambrian is exceedingly rare
5
,
6
. The most widely cited example comprises a single report of morphologically simple mineralized tubes and spheres interpreted as cyanobacteria, obtained from 1,200-Myr-old palaeokarst in Arizona
5
. Organic-walled microfossils were first described from the non-marine Torridonian (1.2–1.0 Gyr ago) sequence of northwest Scotland in 1907
7
. Subsequent studies
8
,
9
,
10
found few distinctive taxa—a century later, the Torridonian microflora is still being characterized as primarily nondescript “leiospheres”
11
. We have comprehensively sampled grey shales and phosphatic nodules throughout the Torridonian sequence. Here we report the recovery of large populations of diverse organic-walled microfossils extracted by acid maceration, complemented by studies using thin sections of phosphatic nodules that yield exceptionally detailed three-dimensional preservation. These assemblages contain multicellular structures, complex-walled cysts, asymmetric organic structures, and dorsiventral, compressed organic thalli, some approaching one millimetre in diameter. They offer direct evidence of eukaryotes living in freshwater aquatic and subaerially exposed habitats during the Proterozoic era. The apparent dominance of eukaryotes in non-marine settings by 1 Gyr ago indicates that eukaryotic evolution on land may have commenced far earlier than previously thought.