BY COLOURING the tree of life according to the highest temperature at which creatures can thrive, scientists have revealed the thermal history of evolution.
The tree shows that a scalding planet gave rise to heat-loving organisms, which gradually adapted to cooler environments as they became more complex. A similar pattern might underlie the evolution of life elsewhere in the universe, the researchers suggest, with life arising almost as soon as temperatures cool enough to allow liquid water, if other basic requirements are met.
Astrobiologists Charles Lineweaver of the University of New South Wales in Sydney and David Schwartzman of Howard University in Washington DC were interested in how the origin and evolution of life on Earth has been affected by temperature. They took as their starting point the most widely accepted evolutionary tree, which was developed in 1997 by Norman Pace of the University of Colorado, Boulder.
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Pace analysed the base sequences of a biological molecule called ribosomal RNA and constructed a tree that shows the relationships between a wide variety of today鈥檚 life forms. It has three main branches, representing bacteria, archaebacteria and the more complex eukaryotes.
The 鈥渞oot鈥� of the tree is the ancestor that gave rise to all life that exists today, thought to have lived around 4 billion years ago. The distances between points on the tree correspond to the number of changes that have occurred in the RNA in that time. So the creatures that are the shortest distance from the root are the most 鈥減rimitive鈥� 鈥� they have evolved the least since life began.
To turn this tree into a thermal record, the researchers looked up the maximum growth temperatures for as many of the species as possible, and coloured the tips of the branches accordingly. Because maximum growth temperature is determined by an organism鈥檚 biochemistry and is unlikely to change suddenly, the researchers extended the colours of the branches back towards the most recent fork. If both branches from a fork were the same colour, they extended the colour back a step further until they had coloured the entire tree (see Diagram).
The result is a visual record of evolution into cooler and cooler niches. Species nearest the root are heat-loving, while more complex creatures further from the root are adapted to cold (). 鈥淵ou can see the evolutionary flow from hot beginnings to cool tips,鈥� says Lineweaver. 鈥淓very creature alive seems to have had ancestors who loved heat more than we do now.鈥� He points out that the pattern is not simply down to the way the tree was created. For example the archaebacterium Archaeoglobus is a rare exception, having apparently re-adapted to warmer temperatures than its ancestors.
Lineweaver speculates that life has always evolved towards ever colder habitats because as the Earth cooled, warmer niches were already filled. In order to spread, life had to adapt to colder and colder temperatures. 鈥淚t鈥檚 intriguing,鈥� says Bruce Weber, an expert on thermal constraints on the origin of life at California State University in Fullerton. 鈥淭he basic correlations the authors use are quite plausible.鈥�
Because the branchings near the root of the tree are dominated by organisms that like it hot, temperature may have been key in determining why life arose when it did, says Lineweaver. 鈥淭hese thermal constraints are excellent candidates for being universal features of life,鈥� he adds, because all planets start hot and then cool down. If life exists elsewhere, 鈥渘ot only will we find that in the beginning it started out simple, but that it started out hot.鈥�