Exploring Origin and evolution of Life
“How far back can we trace the evidence of life on the Earth?”
“How did the building blocks of life emerge on the prebiotic Earth?”
We study the origin, and evolution of life based on the philosophy that “Life was born on the Earth”. Treating the Earth as a model case, we also think about the possibility of life in the universe beyond Earth. This research field is a part of “Astrobiology”. In this field, scientists all over the world explore the origin or existence of life in the universe.
When, where and how was Life on the Earth born?
Where? and How?
For the origin of life, building blocks of life, such as amino acids, nucleobases, sugars were required on the Earth. These molecules would have formed from the atmosphere of the early Earth. The primitive atmosphere is thought to be rich in carbon dioxide, water vapor, and molecular nitrogen. For chemical reactions to synthesize the building blocks of life, those atmospheric components would have experienced reactions with a strong reductant and energy sources.
We hypothesized that iron-rich meteorite impacts on the early ocean could supply the reductant and energy source to synthesize organic molecules on the early earth. We are conducting the “meteorite impact simulation experiments” and “post-impact plume simulation experiments” to reproduce such reactions. As a result, we have succeeded in the synthesis of some organic molecules such as carboxylic acids, glycine, and alanine.
However, only the synthesis of amino acids alone does not lead to the origin of life. We need to polymerize these amino acids to form protein. The polymers of amino acids are called “peptides”. These peptides further polymerizes and then forms a protein.
Amino acids formed by the meteorite impacts would have dissolved into the ocean and precipitated on the ocean floor by being adsorbed clay minerals. In these sediments, amino acids could have been polymerized by dehydration reactions under the high temperature and pressure. On the assumption of this model, we conducted the polymerization experiment of amino acids simulating high temperature and pressure environments in sediments. As a result, we successfully obtained highly polymerized peptides in these conditions.
The research to obtain organic molecules in meteorites is also ongoing. Recently, we successfully detected extraterrestrial sugar from a meteorite. This indicates that sugar can be delivered to the early Earth by the meteorites. Our research is revealing "how the building blocks of life collected on the early Earth".
When? and where?
Where, and from when can the oldest traces of the oldest life on Earth be found? One of them was discovered in 3.7 billion years old of sedimentary rock from the Isua region, Greenland. The “sedimentary rocks” are rocks that were deposited on the ancient seafloor. In 1999, Dr. Rosing of the Royal Danish Institute conducted a chemical analysis on carbon contained in Isua’s sedimentary rocks and reported that life existed in this era.
We collaborated with Dr. Rosing (the person on the right in the photo) to conduct a geological survey in the Isua region and discovered new carbon-containing sedimentary rocks that are traces of life dated to about 3.7 billion years ago. From state-of-the-art analysis such as X-ray crystallography with electron microscopy, we showed evidence that life undoubtedly existed in this era.
How have Life and the Earth been evolved?
The evolution of life that changed the Earth’s environment
How has life evolved into the wide variety of diverse life that we can see today and throughout the history of the Earth? ‘Oxygen’ is one of the keywords for considering the evolution of life.
After the first emergence of life, life has been slowly evolving. It is not clear what did the first organisms used as an energy source. However, at some point, life acquired the ability to use sunlight as energy, known as "photosynthesis". The "oxygenic photosynthesis" that modern plants is utilized in several types of photosynthesis. However, the early Earth has been widely considered to be anoxic before the evolution of oxygenic photosynthesis. By the oxygen production of photosynthetic organisms, the surface of the Earth was filled with oxygen, and life that became more efficient at acquiring energy through aerobic respiration. This allowed life to evolve into larger and more complex form. The rise of oxygen levels brought drastic change in the elemental cycle on the surface of the Earth. An example, is dissolved iron ion in the anoxic ocean, that oxidized and begun to precipitate as iron oxide in the ocean.
So then, when did life start to generate the oxygen that changed ancient life and the global environment? To solve this problem, we are conducting geological surveys around the world, focusing on sedimentary rocks of various geological ages ranging from 3.7 to 1.9 billion years old. We are targeting minerals and organic matter in sedimentary rocks that suggest traces of life, and offer a glimpse of the environment at that time.
Understand ancient life through the modern analogue method
To reveal the origin and evolution of life, the investigation of ancient rocks is indispensable. However, it is always a challenge to reconstruct ancient life and a record of the environment from several billion years old rocks, as they have often suffered complex metamorphisms.
"Modern analogue" is a key idea to understanding ancient life. Microbes living on modern Earth have adopted their "lifestyle" to similar one to that of ancient microbes. One of the popular theories for the origin of life is 'hydrothermal origin'. In the modern hot-spring environment, we can find various microbes in up to 100℃ water. They utilize some chemical components dissolved in the water and after their death, carbonaceous matter or minerals which indicate their lifestyle are left. Careful investigation of such materials tells us the "lifestyle" of microbes in the high temperature environment and we can see the "similarity" with ancient lives through modern examples. This process is called "modern analogue".
We are carrying out the research targeting modern hydrothermal vent and hot springs to understand relationships between microbial activities, precipitated minerals and dissolved component in the water.
The "Kuroko ore", which occurs in Akita prefecture, was formed by ancient hydrothermal activity in 15 million years ago, is also our important target. The Kuroko is like, "fossilized hydrothermal vent". It is not only an important resource for heavy metals (copper, zinc, lead, silver, etc), but also records microbial activity around the hydrothermal vent. We can draw out clues of the "lifestyle" and "environment" of ancient microbes from this section.
'Origin of life on the Earth' is the root of Astrobiology
Could life have been born on the Mars?
In recent years, Mars has attracted attention as a planet where life may exist / have existed. This is because research on Martian meteorites has revealed that it is likely that an ocean and lakes existed on early Mars.
We are researching to clarify if life forming chemical evolution could have occurred in the early Martian environment based on the synthesis conditions of life precursors on the early Earth. As for the early Martian atmosphere, we are conducting joint research with Prof. Terada’s group (Planetary Atmospheric Physics Group, Department of Geophysics, Tohoku University) and estimated the early Martian atmospheric composition by computer simulation based on the conditions inferred from Earth science research.
Furthermore, experiments are being conducted under those estimated conditions. This is one of the examples in how we strive to build a new policy of research that combines geological, theoretical, and experimental knowledge.
image credit: NASA Homepage
Envisioning the future of the Earth.
Think about future of the Earth and life.
The above is research that uncovers the history of how the Earth and life interacted with each other.
It’s carbon that connects the Earth and life tightly. After the emergence of life, the cycle of carbon on the Earth was changed greatly. It must also have been changed alongside the evolution of life. In this change, there existed “rules and laws”.
If we can figure these out, we can predict what will happen to the Earth and life in the future. We are learning from the past.
We, human beings, are an essential factor when envisioning the future of the Earth and life because increased CO2 as a result of human activity are changing the cycle of carbon. The future of the Earth can only be hypothesized on by understanding both past environmental changes and the response of the biosphere to those changes, as well as human impact.
Our research is critical not only for elucidating the unsolved scientific problem of “the origin of life”, but also for the future of the Earth.