YES-27-E-04: Constraining the thermal limits of complex life in a planetary context

Motivation
Microbial life on Earth reproduces within a fairly specific temperature range, roughly -20 to 120ºC. The thermal limits for animal reproduction, however, are even narrower, roughly -2 to 48ºC. Planetary surface temperature therefore constrains not only habitability in the broadest sense, but whether a planet can actively produce and maintain multicellular life like animals. Indeed, some geologists have argued that Earth’s surface environment was too hot to support animals for the majority of Earth’s history. Other geologists have reconstructed dramatic swings in surface temperature – from -50 to 50ºC – during the same interval that animals originated. Astronomers, meanwhile, are trying to recognize habitable planets beyond the Solar System, including planets with surface temperatures supportive of the origin of complex – and even intelligent – life. Much debate and uncertainty persists, however, as we have a poor understanding of how surface temperature controlled the origin of complex life on even our own planet. To better understand how planetary surface temperatures permit (and prevent) the origin of complex life, the temperature window for the origin of animal-like multicellularity on Earth needs to be experimentally constrained.

Project Background
While the origin of animal multicellularity on Earth was a historical event, and the earliest animals have long been extinct, the temperatures associated with animal origins can be reasonably inferred using modern organisms. Choanoflagellates – small, bacteria-eating microbial eukaryotes – are the closest living relatives of animals, and commonly alternate between unicellular and multicellular phenotypes. By experimentally determining the upper and lower temperature thresholds for multicellular development across different choanoflagellate species, the temperature range associated with the origin of animal life can be empirically constrained. These constraints can then inform Earth system models reconstructing the co-evolutionary history of life and Earth’s surface temperature, as well as efforts to characterize the habitability of Earth-like exoplanets.

Project Overview
With this planetary framework in mind, this project will primarily involve culturing different species of colonial choanoflagellates in the laboratory and quantifying their growth (via light microscopy) at different temperatures, spanning roughly 5 to 55ºC. The goal is to quantify the maximum, minimum, and optimum temperatures for growth (division) for different choanoflagellate species, as well as quantifying the relationship between temperature and multicellular colony formation. For example, are there temperatures below and above which colony formation ceases across choanoflagellates, but unicellular growth continues? At which temperature is colony formation most favored? How variable or conserved are these different thermal tolerances across different choanoflagellate species? Answering these questions will allow us to isolate the role of temperature during the earliest stages of animal-like multicellularity – from the temperatures most conducive to this major evolutionary transition to the temperatures that likely exclude it from happening altogether. This project will further involve opportunities to place these experimental results into Earth system models, as well as meta-analyses designed to help determine which kinds of exoplanets should be prioritized in the search for complex life beyond Earth.