Introduction
The appearance of the eukaryotes (all organisms that have cells in which the nucleus is enclosed within a membrane) represents one of the most fundamental milestones in the history of life (Butterfield, 2015). This transition occurred when the eukaryotes split off from the prokaryotic Archaea by acquiring the intra-cellular organization and other features that distinguishes them from the prokaryotes. The process by which this transformation occurred remains unclear (Galbaldón and Pittis, 2015).
In order to place the appearance of eukaryotes in the context of geological time, we need to be able to find evidence of their existence in the rocks. Molecular biomarkers and isotopes have been invoked, but for the eukaryotes do not provide evidence for an earlier origin than do physical fossils (Javaux and Lepot, 2018).
Recognition of eukaryotes in the fossil record requires recognition of the synapomorphies associated with the Eukaryota. Unfortunately, the known synapomorphies of the eukaryotes are defined almost exclusively in terms of what can be observed inside extant cells (Fehling et al, 2007). Detecting those features in ancient fossils is extremely difficult or impossible. For this reason, other characteristics have been suggested, such as large size (40-250μm) and complex wall structure including striations, longitudinal ruptures, and a trilaminar organization (Betts et al, 2018). Porter (2020) states that “the oldest widely accepted evidence of eukaryotes is large (greater than 100 µm), spiny, ornamented, organic walled microfossils found in latest Paleoproterozoic rocks (ca 1650 Ma)”.
Using these criteria, it is possible to recognize eukaryotic microfossils in the fossil record. However, it is not possible to say whether they belong to the stem group or the crown group, and so they can be considered only as members of the total group of the Eukaryota (Betts et al, 2018).
In order to place the appearance of eukaryotes in the context of geological time, we need to be able to find evidence of their existence in the rocks. Molecular biomarkers and isotopes have been invoked, but for the eukaryotes do not provide evidence for an earlier origin than do physical fossils (Javaux and Lepot, 2018).
Recognition of eukaryotes in the fossil record requires recognition of the synapomorphies associated with the Eukaryota. Unfortunately, the known synapomorphies of the eukaryotes are defined almost exclusively in terms of what can be observed inside extant cells (Fehling et al, 2007). Detecting those features in ancient fossils is extremely difficult or impossible. For this reason, other characteristics have been suggested, such as large size (40-250μm) and complex wall structure including striations, longitudinal ruptures, and a trilaminar organization (Betts et al, 2018). Porter (2020) states that “the oldest widely accepted evidence of eukaryotes is large (greater than 100 µm), spiny, ornamented, organic walled microfossils found in latest Paleoproterozoic rocks (ca 1650 Ma)”.
Using these criteria, it is possible to recognize eukaryotic microfossils in the fossil record. However, it is not possible to say whether they belong to the stem group or the crown group, and so they can be considered only as members of the total group of the Eukaryota (Betts et al, 2018).
Total-Eukarota
The oldest known members of the eukaryote total group are organic-walled microfossils from the late Paleoproterozoic (Statherian) Changcheng Group in the Yanshan Range, North China (Miao et al, 2019). Some of these are illustrated below (click on image for larger view):
Crown-Eukaryota
The oldest fossil that can be confidently assigned to the crown group of the eukaryotes is Bangiomorpha pubescens, a red alga (total-Rhodophyta) from the Mesoproterozoic (Ectasian) to Neoproterozoic (Tonian) Lower Hunting Formation, of Somerset Island, Arctic Canada (Betts et al, 2018). The only image available in the public domain is shown below:
Comparison of the ages of the oldest total-group eukaryotes and Bangiomorpha pubescens indicates that there is a period of uncertainty of at least 369 million years (between 1,638 and 1,269 million years ago) for the time of appearance of the eukaryote crown group.
References
Betts, H. C., Puttick, M. N., Clark, J. W., Williams, T. A., Donoghue, P. C., & Pisani, D. (2018). Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origin. Nature ecology & evolution, 2(10), 1556-1562.
Butterfield, N. J. (2015). Early evolution of the Eukaryota. Palaeontology, 58(1), 5-17.
Fehling, J., Stoeker, D., & Baldauf, S. L. (2007). Photosynthesis and the eukaryote tree of life. In Evolution of primary producers in the sea (pp. 75-107). Academic Press.
Gabaldón, T., & Pittis, A. A. (2015). Origin and evolution of metabolic sub-cellular compartmentalization in eukaryotes. Biochimie, 119, 262-268.
Javaux, E. J., & Lepot, K. (2018). The Paleoproterozoic fossil record: implications for the evolution of the biosphere during Earth's middle-age. Earth-Science Reviews, 176, 68-86.
Miao, L., Moczydłowska, M., Zhu, S., & Zhu, M. (2019). New record of organic-walled, morphologically distinct microfossils from the late Paleoproterozoic Changcheng Group in the Yanshan Range, North China. Precambrian Research, 321, 172-198.
Porter, S. M. (2020). Insights into eukaryogenesis from the fossil record. Interface Focus, 10(4), 20190105.
Butterfield, N. J. (2015). Early evolution of the Eukaryota. Palaeontology, 58(1), 5-17.
Fehling, J., Stoeker, D., & Baldauf, S. L. (2007). Photosynthesis and the eukaryote tree of life. In Evolution of primary producers in the sea (pp. 75-107). Academic Press.
Gabaldón, T., & Pittis, A. A. (2015). Origin and evolution of metabolic sub-cellular compartmentalization in eukaryotes. Biochimie, 119, 262-268.
Javaux, E. J., & Lepot, K. (2018). The Paleoproterozoic fossil record: implications for the evolution of the biosphere during Earth's middle-age. Earth-Science Reviews, 176, 68-86.
Miao, L., Moczydłowska, M., Zhu, S., & Zhu, M. (2019). New record of organic-walled, morphologically distinct microfossils from the late Paleoproterozoic Changcheng Group in the Yanshan Range, North China. Precambrian Research, 321, 172-198.
Porter, S. M. (2020). Insights into eukaryogenesis from the fossil record. Interface Focus, 10(4), 20190105.
Image credits - Eukaryotes
- Header: Amanita muscaria toadstool By Onderwijsgek at nl.wikipedia / CC BY-SA 3.0 NL (https://creativecommons.org/licenses/by-sa/3.0/nl/deed.en)
- All except Bangiomorpha pubescens are from Open Access article Loron, C. C., Rainbird, R. H., Turner, E. C., Greenman, J. W., & Javaux, E. J. (2019). Organic-walled microfossils from the late Mesoproterozoic to early Neoproterozoic lower Shaler Supergroup (Arctic Canada): diversity and biostratigraphic significance. Precambrian Research, 321, 349-374.
- Bangiomorpha pubescens: from Open Access article Garwood, R. J. (2012). Patterns In Palaeontology: The first 3 billion years of evolution. Palaeontology Online, 2(11), 1-14.