The timeline of terrestrial evolution holds surprises. While scientists thought complex life required oxygen, a recent discovery shows it began forming in oceans deprived of this element, nearly a billion years earlier than estimated.
The first organisms on Earth were prokaryotes, simple cells without a nucleus. For billions of years, they dominated the planet. It was only much later that eukaryotes, with their elaborate internal structures, appeared. This group includes algae, fungi, plants, and animals.
To trace this history, an international team used an improved version of the molecular clock (see the explanation at the end of the article). This technique estimates the time when species shared a common ancestor. By analyzing over one hundred gene families and comparing them to fossil data, the researchers reconstructed a more precise tree of life. The study, led by scientists from the University of Bristol, combines paleontology, phylogenetics, and molecular biology to offer a detailed view.
The results are astonishing. The transition towards more elaborate cells would have begun approximately 2.9 billion years ago. Structures like the nucleus formed well before the appearance of mitochondria. These conclusions led to a new model named 'CALM', for Complex Archaeon, Late Mitochondrion. The researchers explain that this scenario replaces previous hypotheses about eukaryogenesis.
An important aspect of this study is the time gap between the evolution of eukaryotes and the rise of atmospheric oxygen (more details at the end of the article). Mitochondria, often associated with oxygen respiration, appeared later, coinciding with the first significant increase in oxygen. Thus, the archaeal ancestor of eukaryotes developed complex traits in an anaerobic environment. This indicates that oxygen was not a prerequisite for the initial stages of this evolution.
This research, published in
Nature, challenges several established ideas. It demonstrates that life could evolve into advanced forms without immediately depending on oxygen. The implications are vast, opening new perspectives on the conditions conducive to the emergence of biological complexity.
The analysis of over one hundred gene families helped reconstruct the developmental pathway of complex life, highlighting the differences between prokaryotes and eukaryotes.
Credit: Dr. Christopher Kay The molecular clock
The molecular clock is a technique used in evolutionary biology to estimate the time elapsed since two species diverged from a common ancestor. It is based on the idea that genetic mutations accumulate at a relatively constant rate over time. By comparing DNA or protein sequences between different species, scientists can calculate approximate dates for evolutionary events.
This method requires calibration data, often drawn from known fossils. For example, if a fossil of an organism is dated to a certain age, this allows adjusting the mutation rate. The recent study expanded this approach by integrating hundreds of species and focusing on specific gene families, thus improving the accuracy of the estimates.
The molecular clock is particularly useful for studying periods where fossils are scarce, such as the early evolution of life. It helps fill gaps in the fossil record and reconstruct more complete phylogenetic trees. However, it has limitations, as the mutation rate can change depending on lineages or environmental conditions.
Recent advances, such as the use of elaborate statistical models, have made this technique more reliable. It continues to play an essential role in our understanding of the history of life on Earth, enabling discoveries that redefine established timelines.
The role of oxygen in evolution
Traditionally, atmospheric oxygen was considered an important element for the emergence of complex life. It was thought that high levels of oxygen, reached about 2.4 billion years ago during the Great Oxidation Event, were necessary for the development of eukaryotes, particularly to power mitochondria via cellular respiration.
However, the new study reveals that eukaryotic characteristics appeared well before this period, in environments devoid of oxygen. This indicates that oxygen was not an initial driver of evolution towards more elaborate cells. Ancestral archaea were able to evolve into more advanced forms by using other energy sources, such as chemical compounds available in the primitive oceans.
This perspective changes our view of conditions conducive to life. It shows that biological complexity can emerge under diverse metabolic regimes, without exclusively depending on oxygen. This has implications for research into the origins of life on Earth and potentially on other planets, where anaerobic environments could also favor evolution.