The Last Universal Common Ancestor: LUCA

When discussing biological classification, it is often stated that a natural taxonomic group should be monophyletic. This means that all species within that group share a single common ancestor, and all descendants of that ancestor belong to the same group.

This raises an interesting question: do all living organisms on Earth form a single monophyletic group?

Although direct fossil evidence has not yet confirmed this, the theoretical answer is yes. The earliest member of this monophyletic group—the common ancestor from which all modern organisms ultimately descended—is referred to as the Last Universal Common Ancestor, commonly abbreviated as LUCA.

LUCA is estimated to have lived around 4.2 billion years ago. From this ancestral organism, life eventually diverged into major lineages such as bacteria and archaea, which subsequently evolved into the enormous diversity of life present on Earth today.

It is important to note, however, that LUCA was probably not the earliest living organism, nor was it the only life form existing at the time. Many other early organisms likely existed but eventually went extinct without leaving descendants that survive today. LUCA itself must also have had ancestors, but because those organisms are not the last common ancestor shared by all modern life, they are not included within the monophyletic group of living organisms.

In one study, researchers compared the genomes of 350 bacterial species and 350 archaeal species, searching for shared genetic features in order to infer which genes LUCA might have possessed and to estimate when the divergence between these lineages occurred.

Based on current research, several characteristics of LUCA have been proposed.

1. A relatively large genome: Studies suggest that LUCA possessed a genome larger than previously assumed. It may have encoded at least 2,600 proteins, a complexity comparable to that of modern prokaryotes.

2. An anaerobic organism living in shallow marine environments: LUCA likely thrived in oxygen-free environments and possessed genes capable of protecting it from ultraviolet radiation. It also appears to have had a basic immune system. These features suggest that LUCA may have lived in shallow marine environments, using hydrogen and carbon dioxide as energy sources. The presence of immune-related genes further indicates that viruses were already present in the environment, engaging in evolutionary competition with early microbes such as LUCA and contributing to the formation of an early but already complex ecosystem.

3. A thermophilic organism: Genetic evidence indicates that LUCA possessed genes commonly associated with thermophilic microorganisms. This suggests that, in addition to inhabiting shallow marine environments, LUCA may also have lived near submarine volcanic regions or hydrothermal systems.

（圖片來源：Chiswick Chap，採用  CC BY-SA 4.0 授權。）

Although many aspects of LUCA remain uncertain, ongoing research continues to refine our understanding of this pivotal ancestor. By reconstructing the biology of LUCA, scientists gain valuable insights into the earliest stages of life and the evolutionary origins of all organisms living today.

Author: Bai Leng

Reference:

Moody, E. R. R., Álvarez-Carretero, S., Mahendrarajah, T. A., Clark, J. W., Betts, H. C., Dombrowski, N., Szánthó, L. L., Boyle, R. A., Daines, S., Chen, X., Lane, N., Yang, Z., Shields, G. A., Szöllősi, G. J., Spang, A., Pisani, D., Williams, T. A., Lenton, T. M., Donoghue, P. C. J. (2024). The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution.