news & views PALAEOBIOLOGY Valuable snapshots of deep time A regional oxygenation event 1.6 billion years ago coincided with the appearance of large fossils, but whether the availability of oxygen was the primary driver of the diversification of multicellular organisms remains to be seen. Emma U. Hammarlund A classic question in Earth history is why large life forms took so long to make a widespread appearance. Large forms of life only genuinely challenged unicellular life after a 4-billion- year walkover. Animals and macroalgae diversified dramatically some 600–500 million years ago, but no coherent tale is preserved of what led up this event. We are left with snapshots, where every recovered corner of an image is precious. To merge these precious corners into one sharp image, however, is challenging. Each curator emphasizes a different aspect of the snapshots, with geochemists seeking chemical hints to ancient ocean conditions and palaeobiologists scrutinizing the preserved shapes for signs of life. The perhaps most shared working hypothesis among both groups has been that snapshots of oxic conditions and large life will never pile up from the billion-year era prior to diversification. Indications of oxic conditions and reports of putative macrofossils suggest that this position may have to change. Writing in Nature Geoscience, Zhang and co-authors 1 add to this evidence, reporting a shift from anoxic to oxic water column conditions 1.6 billion years ago, in association with the occurrence of peculiar large fossils. Oxygen is a suspected accomplice to the evolution of complex eukaryotes, as it amplifies life’s ability to build biomass. Although simple eukaryotes appear somewhat indifferent to oxygen 2 , most animals absolutely require this gas. Somewhere between simple and complex eukaryotes, oxygen must have gotten a grip on the evolutionary storyline. We struggle to understand when and how. Evidence of free oxygen during the interval from 2.5–0.7 billion years ago (Ga) exists, such as in shallow settings at about 1.8 Ga (ref. 3 ), within local mats around 1.6 Ga (ref. 4 ), deeper on the shelf at approximately 1.4 Ga (refs 5,6 ) and even globally at about 1.1 Ga (ref. 7 ). However, it is unclear whether these oxygen levels would be sufficient for complex eukaryotes, whose requirements remain poorly constrained. In parallel, fossils of large, eukaryotic, complex organisms — way too old for our conventional understanding of their evolution — have been found, such as fungal hyphae from 2.4 Ga (ref. 8 ) and red algae from 1.6 Ga (ref. 9 ). At face value, these fossils might indicate that oxygen levels were sufficient for complex eukaryotes, except that the eukaryotic affinity of these fossils remains contested. Among these putative fossil eukaryotes are leaf-like algae from the approximately 1.6-billion-year- old Gaoyuzhuang Formation 10 (Fig. 1). These fossils are huge, relatively speaking, consisting of decimetre-long lanceolate- shaped compressions, and consist of sheets of tissue with nicely packed cells 9 . Zhang et al. 1 delve into the chemistry of the rocks surrounding these fossils in order to assess the environmental conditions they (1) (2) a b 4.6 2.3 0.6 0 (Ga) Small life Large life Free O 2 Gaoyuzhuang Fig. 1 | Mesoproterozoic macrofossils and indications of free oxygen. a, The 1.6-billion-year-old fossils were identified in the Gaoyuzhuang basin, demonstrating: (1) linear shapes; and (2) tongue shapes with longitudinal striations. Image reproduced from ref. 10 , Macmillan Publishers Ltd. b, A simplified overview of our current observations of free oxygen, large life (complex eukaryotes) and small life (prokaryotes and simple eukaryotes) over Earth's history (for example, see refs 2–9 ). The Gaoyuzhuang basin is marked with a box (dashed) including the current contribution by Zhang et al. 1 (orange). NATURE GEOSCIENCE | www.nature.com/naturegeoscience © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.