The evolution of complex life and the stabilization of the Earth system

Abstract

The half-billion-year history of animal evolution is characterized by decreasing rates of background extinction. Earth's increasing habitability for animals could result from several processes: (i) a decrease in the intensity of interactions among species that lead to extinctions; (ii) a decrease in the prevalence or intensity of geological triggers such as flood basalt eruptions and bolide impacts; (iii) a decrease in the sensitivity of animals to environmental disturbance; or (iv) an increase in the strength of stabilizing feedbacks within the climate system and biogeochemical cycles. There is no evidence that the prevalence or intensity of interactions among species or geological extinction triggers have decreased over time. There is, however, evidence from palaeontology, geochemistry and comparative physiology that animals have become more resilient to an environmental change and that the evolution of complex life has, on the whole, strengthened stabilizing feedbacks in the climate system. The differential success of certain phyla and classes appears to result, at least in part, from the anatomical solutions to the evolution of macroscopic size that were arrived at largely during Ediacaran and Cambrian time. Larger-bodied animals, enabled by increased anatomical complexity, were increasingly able to mix the marine sediment and water columns, thus promoting stability in biogeochemical cycles. In addition, body plans that also facilitated ecological differentiation have tended to be associated with lower rates of extinction. In this sense, Cambrian solutions to Cambrian problems have had a lasting impact on the trajectory of complex life and, in turn, fundamental properties of the Earth system.

Keywords: animals; biodiversity; evolution.

Figure 1.

Time scale for the history of the Earth, with key events in the history of life. Diversity of animal life (marine animal families) plotted on the vertical axis (from [1]). Colour differences in the Proterozoic and Phanerozoic bars indicate era boundaries within the eons. Figure inspired by an unpublished figure belonging to W. Fischer.

Figure 2.

Phanerozoic records from the fossil record and geochemistry of marine sediments. (a) Per capita extinction rate, illustrating a long-term decline in the background extinction rate punctuated by mass extinction events (data from The Paleobiology Database). Also plotted are the timing of major ice ages, eruptions of large igneous provinces and the largest known impact events. (b) Mean (log-transformed) body size of marine animals, illustrating a long-term increase in mean body volume totals more than two orders of magnitude (replotted from [7]). (c) Total number of ecological modes occupied by marine animals, illustrating the long-term increase in the number of modes of life present in the oceans (replotted from [8]). (d) I/Ca ratio of marine sedimentary carbonates (replotted from [9]), illustrating a progressive increase in I/Ca of marine carbonates, implying an increase in oxygen concentrations in typical ocean surface waters. (e) Carbon isotope composition of marine carbonate sediments (replotted from [10]), illustrating a decrease in the magnitude of carbon isotope variation between the Cambrian and the present day.

Figure 3.

Plots of genus richness versus the number of ecological modes of life for the Late Ordovician and Pleistocene. While the number of genera and modes within a Linnaean class were not correlated after the Cambrian and Ordovician radiations, they are correlated today. The strong correlation between Ordovician and Pleistocene ecological modes within classes indicates that classes that were able to diversify into many modes early in their histories have remained the most ecologically diverse and have also become the most taxonomically diverse [51] (plotted all classes with at least five genera in both the Ordovician and Pleistocene). Data from Knope et al. [8].