From snowball Earth to the Cambrian explosion: Evidence from China
Geological Magazine Guest Editor, Xian-Hua Li answers questions on the thematic issue “From Snowball Earth to the Cambrian Explosion: Evidence from China”.
Q: What does “Snowball Earth” mean?
A: Snowball Earth means that Earth was covered by ice entirely (from poles to equator) for long periods of time in the geological past. Notably two Snowball Earth episodes (Sturtian and Marinoan) were separated by a super-hot interglacial interval during the Cryogenian Period (720–635 Ma). It seems that the emergence of complex multicellular life was closely related in time to the Snowball Earth. Thus, it has been an attractive question in science: whether Snowball Earth kick-started the evolution of life, and in particular the animals?
Q: What is the “Cambrian Explosion”?
A: The “Cambrian Explosion” is a metaphor for the sudden appearance of nearly all metazoan (all animals except sponges) body plans (or phyla) during early Cambrian time (ca. 540–518 Ma). It remains a scientific puzzle that was brought to public awareness through Charles Darwin’s great book On the Origin of Species (1859), and may represent the most important evolutionary event in the history of life on Earth.
Q: Why is China so critical in this research field?
A: China, particularly South China, preserves a complete stratigraphic succession with extraordinarily well-preserved, successive fossil biotas (see Fig. 1 in Introduction of this thematic issue) during the time interval from the Snowball Earth to Cambrian Explosion (720–518 Ma). These fossils biotas, including phosphatized microscopic animal fossils with cellular details from the Doushantuo phosphorites in Weng’an (Guizhou; ca. 609 Ma) and the soft-bodied animals (see trilobite with well-preserved appendages described by Zeng et al. in this thematic issue, p.1306) from the Maotishan Shale in Chengjiang (Yunnan; ca. 518 Ma), provide unique archives of early animal evolution. One of these sites includes the Chengjiang fossil site that is placed on the UNESCO World Heritage List (2012). Furthermore,The lithologically composite stratigraphic successions of South China provide ample potential for application of various sedimentary geochemical proxies for documenting environmental changes. Because of all these reasons, South China has become a key area and global focus of studies in the field over recent decades.
Q: When did animals appear on the Earth?
A: The sponge molecular fossils and the sponge-like body fossils before the end of the Snowball Earth seem to indicate that the animals appear on the Earth earlier than 635 million years ago. However, these fossils remain in debates, as Luo et al. argue that sponge-like body fossils before 635 Ma may actually be a result from microbial activities (in this thematic issue, p.1269). So far, the microscopic animal fossils with cellular details from the Doushantuo phosphorites and cherts of South China represent the most convincing and earliest animal fossils (see thumbnail image of the thematic issue). The isotopic dating of the fossil-bearing rocks suggests that the animals appear on the Earth older than 609 million years ago (see Zhou et al. this thematic issue, p. 1193).
Q: What triggered the “Cambrian Explosion”?
A: This remains a puzzling question! Increasing evidence suggests that the Cambrian Explosion was directly linked to increased oxygenation of the atmosphere and ocean during that time. However, the exact timing and mechanism of the oxygenation event, and how it links to the early evolution of animals, remain elusive. Recent multi-proxy geochemical studies from South China have indicated a stepwise increase of oxygenation during the Ediacaran and early Cambrian, and that modern-like oxygen levels were reached for the first time in Earth history at 521 Ma, coinciding with the rapid radiation of all animal forms seen in fossil records from South China. Meanwhile, other studies have proposed a more dynamic history of redox evolution during the Ediacaran and early Cambrian, suggesting that regional heterogeneity of redox conditions through time may have exerted a strong impact on the early evolution of animals (see Wei et al., p.1344 & Jin et al., p.1360 this thematic issue).
The “Introduction: from snowball Earth to the Cambrian explosion–evidence from China” by Maoyan Zhu and guest editor Xian-Hua Li is available to download free of charge for a limited time here.