Head et al. present a timely cautionary note regarding Franz and colleagues’ report of eukaryotic fossils preserved within 1.5-1.7 billion year old pegmatite. As Head et al. make clear, the fact that microorganisms occur in cracks, vesicles or other spaces spaces within 1.5 billion year old rocks does not mean that the microbes themselves are 1.5 billion years old.  As studies of Earth’s deep biosphere document, living microorganisms can be found today in many subterranean environments, including igneous and metamorphic substrates. We can debate questions of fungal antiquity, but Head et al. are unambiguously correct that the reported microstrcutures include vascular plant hairs and pine pollen, which must be much younger than the rocks in which they occur.   The filaments with apparent sporangia are probably fungi, but as they are very likely the same age as the plant materials, they, too, are almost certainly much younger than the pegmatites.

Head et al. helpfully place the reported fossils within a broader physical and biological context.  Whether the assembly of Rodinia records the onset of modern plate tectonics is hotly debated; recent papers by Chris Spencer may provide a more nuanced sense of the tectonic state of the mid-Proterozoic Earth.  Also, note in line 66 that Gibson et al. dated the oldest known red algae and not the advent of photosynthesis within eukaryotes, which must have evolved earlier in stem group archaeplastids.  The fossil record of fungi themselves is fraught.  Loron et al. discovered interesting filaments in billion year old rocks, but the presence of sporangia at the termini of filaments does not make the fossils unambiguously fungal.  (Neither does the inferred wall chemistry.)  They could be, but calling them “possible” fungi would be recommended.  And Head et al should note that Bengtson et al.’s report of mycelia in 2.4 billion year old basalt vesicles is subject to the same issues that surround Franz et al.’s structures.  It might be noted that no molecular clock for fungal evolution predicts anything close to 1.5 billion year old fungi.  While one might argue that the fungal fossil record is poorly sampled and may therefore yield erroneously young estimates of divergence time, molecular clocks for animals – close relatives of fungi – also hover around 700-800 million years, consistent, at least, with published clocks for fungi.

These are all details, but Head et al.’s identification of vascular plant remains in the Volyn rocks unambiguously shows that Volyn pegmatites contains biological materials much younger than the rocks themselves. The hypothesis that Volyn structures could have been emplaced as dust permeating museum drawers is both reasonable and testable.  The description of what turn out to be modern remains in ancient rocks was common in the early days of Precambrian paleontology, but is relatively rare today.  Nonetheless, Head et al. provide a timely reminder that care is always advisable, especially when reporting stratigraphically unexpected remains from geologically unusual substrates.

The comment by Head and colleagues presents a welcome reminder of the challenges of analysing early traces of life in rocks (syngenicity and biogenicity). The comment is well structured, clear to read and highlights the main issues arising from Franz et al 2023 study. I concur that the illustrated objects are unambiguous modern contaminants.

The section on FTIR spectroscopy is very pertinent. Possibly, the authors could emphase more on the fact that the reported bands (CH stretching and bending for example),  are very general organic band that could not by themselves constitute a strong reference point. The general aspect of the original spectra is indeed very characteristic of spectra for bitumen, kerogen or mature sedimentary organic carbon in general. The high ratio between CH3 and CH2, the intense and broad absorption around 1660 cm-1, and the hydrothermal influence makes me think that this organic material is quite degraded, with short chain aliphatic and strong aromatic contribution. 

I recommend this comment for publication.

Reports of well preserved, early Mesoproterozoic eukaryotes in pegmatite cavities from the Volyn region of Ukraine (Franz et al. 2023) are extraordinary, and demand a corresponding level of interrogation – for biogenicity, eukaryoticity, and of course syngenicity with the host deposit.  Head et al. take an important first pass at scrutinizing these putative microfossils, drawing immediate attention to a specimen that looks very much like a bisaccate pollen grain.  The comparison is compelling and legitimately introduces the likelihood of modern contamination.  It’s possible, of course, that the similarity is entirely accidental – a pareidolia – but difficult to maintain alongside other constituents that look very much like modern plant trichomes and other airborne debris.  In this context, there is little point in debating the nature of associated geochemical signatures or broader phylogenetic affiliations.  The first order of business is identifying which, if any, of these structures is of Proterozoic age?

The host material is a fibrous bituminous-kerogenous ‘kerite’ deposit that was emplaced at some point prior to ~1500 Ma.  Importantly, its dark/opaque appearance under white light (Franz et al. 2023, fig. 2a-c) clearly distinguishes its constituent fibres from ‘museum dust.’  That’s not to suggest that all of the recoverable fibres are Proterozoic – any more than filamentous microbes in the Nakhla meteorite necessarily come from Mars (cf., Toporski & Steele 2007) – but there are clearly indigenous structures in the Volyn kerite that warrant consideration.  At the same time, it’s worth appreciating the likely biological origin of the carbon from which the kerite was formed.  Whatever its particular mode of emplacement, this will almost certainly be transmitting inherited biogeochemical signatures.  The real question then, is whether any of the demonstrably indigenous Volyn structures are demonstrably biogenic in terms of their current morphology? 

Branching fibrous structures recovered from the Volyn kerite share a number of features with modern filamentous fungi (Franz et al. 2023, figs. 3a-i, 4-6), but none that are diagnostically fungal.  At a more fundamental level, these filaments also fail to satisfy one of the key criteria for assessing first-order biogenicity – namely a hollow constitution that convincingly mirrors the distinction between a structural, carbon-rich, cell wall and its essentially water-filled lumen (Buick 1990).  Early diagenetic mineralization commonly prevents microfossil collapse, but it remains to be seen how such a process could produce the massive kerogenous interiors of the Volyn fibres.  The more likely origin of such structures is through abiological processing, associated with the emplacement and hydrothermal alteration of locally ponded bitumen.  To be fair, Franz et al (2023, p. 1920) do address the possibility of abiological biomorphs, but with no serious interrogation of any carbon-based ‘chemical garden’ models (cf., Lukjanova & Lovzova 1994, McMahon 2019).

Buick, R. 1990. Microfossil recognition in Archean rocks: an appraisal of spheroids and filaments from a 3500 m.y. old chert-barite unit at North Pole, Western Australia. Palaios 5, 441–459.

Franz, G., Khomenko, V., Lyckberg, P., Chournousenko, V., Struck, U., Gernert, U. & Nissen, J. 2023. The Volyn biota (Ukraine) – indications of 1.5 Gyr old eukaryotes in 3D preservation, a spotlight on the “boring billion”. Biogeosciences 20, 1901–1924.

Lukjanova, V.T. & Lovzova, R.V. 1994. Carbon fibers in nature. Carbon 32, 777–783.

McMahon, S. 2019. Earth’s earliest and deepest purported fossils may be iron-mineralized chemical gardens. Proceedings of the Royal Society B, 286, 20192410.

Toporski, J. & Steele, A. 2007. Observations from a 4-year contamination study of a sample depth profile through martian meteorite Nakhla. Astrobiology 7, 389–401.