Earth's Climate History Explains Life's Temperature Optima

Abstract

Why does the growth of most life forms exhibit a narrow range of optimal temperatures below 40°C? We hypothesize that the recently identified stable range of oceanic temperatures of ~5 to 37°C for more than two billion years of Earth history tightly constrained the evolution of prokaryotic thermal performance curves to optimal temperatures for growth to less than 40°C. We tested whether competitive mechanisms reproduced the observed upper limits of life's temperature optima using simple Lotka-Volterra models of interspecific competition between organisms with different temperature optima. Model results supported our proposition whereby organisms with temperature optima up to 37°C were most competitive. Model results were highly robust to a wide range of reasonable variations in temperature response curves of modeled species. We further propose that inheritance of prokaryotic genes and subsequent co-evolution with microbial partners may have resulted in eukaryotes also fixing their temperature optima within this narrow temperature range. We hope this hypothesis will motivate considerable discussion and future work to advance our understanding of the remarkable consistency of the temperature dependence of life.

Keywords: early evolution; selection pressure; temperature optima.

FIGURE 1

Earth's temperature history determined life's thermal optima. We propose that stable ocean temperatures with an upper limit of ~37°C since the Archean (T _maxE) imposed strong selection pressures on the temperature optima of early life via thermal constraints on cell structure, metabolism, and resource acquisition. These macroevolutionary pressures likely gave rise to the observed universal upper limit of temperature optima (T _opt) in modern lifeforms.

FIGURE 2

Organisms receive a competitive advantage when optimizing growth rates to local environmental temperatures. We propose that selection was driven by competitive advantages of increased growth rates for organisms with T _opt that closely match environmental temperatures due to opposing upward and downward selection pressures on organismal temperature optima (A). Simple competition models (B) suggest organisms with lower T _opt mainly face competitive constraints, whereas organisms with the highest T _opt only escape competition at warmer extremes but eventually face strong maintenance costs and death as they approach upper critical temperatures for growth (T _max). Inset in (B) shows the competitive dynamics of three model species at an environmental temperature of 37°C.

FIGURE A1

Growth rate–temperature relationships used for the competition model in Figure 1.3.

FIGURE A2

Competition of all three species at 37°C.

FIGURE A3

Competition of S1 and S2 at 37°C.

FIGURE A4

Competition of S1 and S2 across 25.0°C–45.0°C.

FIGURE A5

Competition of S1 and S3 at 37°C.

FIGURE A6

Competition of S1 and S3 across 25.0°C–45.0°C.

FIGURE A7

Growth rate–temperature relationships without normalization.

FIGURE A8

Competition of species at 37°C without normalization of the growth rate–temperature relationships.

FIGURE A9

Competition of species across 25.0°C–45.0°C without normalization of the growth rate‐temperature relationships.

FIGURE A10

Growth rate–temperature relationships without differences in skew.

FIGURE A11

Competition of species at 37°C without differences in the skew of the growth rate–temperature relationships.

FIGURE A12

Competition of species across 25.0°C–45.0°C without differences in skew of the growth rate–temperature relationships.

FIGURE A13

Competition of species across 25.0°C–45.0°C with skewedness of 0.0 for all of the growth rate–temperature relationships.

FIGURE A14

Competition of species across 25.0°C–45.0°C with skewedness of −2.5 for all of the growth rate–temperature relationships.

FIGURE A15

Competition of species across 25.0°C–45.0°C with skewedness of −5.0 for all of the growth rate–temperature relationships.

FIGURE A16

Competition of species across 25.0°C–45.0°C with skewedness of −7.5 for all of the growth rate–temperature relationships.

FIGURE A17

Competition of species across 25.0°C–45.0°C with skewedness of −10.0 for all of the growth rate–temperature relationships.

FIGURE A18

Competition of species across 25.0°C–45.0°C with skewedness of −15.0 for all of the growth rate–temperature relationships.

FIGURE A19

Competition of species across 25.0°C–45.0°C with skewedness of −20.0 for all of the growth rate–temperature relationships.

FIGURE A20

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 0.1°C.

FIGURE A21

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 0.5°C.

FIGURE A22

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 1.0°C.

FIGURE A23

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 2.0°C.

FIGURE A24

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 5.0°C.

FIGURE A25

Competition of species across 25.0°C–45.0°C with differences in the temperature optimum of 10.0°C.