ゲノム配列の比較から明らかになった初期生命の好熱性/Thermophilicity of early life revealed by comparing extant genomes

Abstract

Understanding the origin and early evolution of life is fundamental to improve our knowledge on ancient living systems and their environments. Information about the environment of early Earth is sometimes obtained from fossil records. However, no fossil records of ancient organisms that lived more than 3,500 million years ago have been found. Instead, we can now predict the sequences of ancient genes and proteins by comparing extant genome sequences accumulated by the genome project of various organisms. A number of computational studies have focused on ancestral base contents of ribosomal RNAs and the amino acid compositions of ancestral proteins, estimating the environmental temperatures of early life with conflicting conclusions. On the other hand, we experimentally resurrected inferred ancestral amino acid sequences of nucleoside diphosphate kinase that might have existed 3,500–3,800 million years ago. The resurrected proteins are stable around 100℃, being consistent with the thermophilic ancestry of life. Our experimental data do not exclusively indicate the thermophilic origin of life; rather, our conclusion is compatible with the idea that the hyperthermophilic ancestor was selected for increased environmental temperatures of early Earth probably caused by meteorite impacts.