**Article 21** **Manifestations of Early Life on Earth** 1. **Definition:** * Early life science is the study of the origins and early evolution of life on Earth. * It aims to understand the conditions that led to the emergence of the first living organisms and how they evolved over billions of years.

2. **Early Earth:** * Earth formed about 4.5 billion years ago. * The early conditions of Earth were very harsh: * Very high temperatures. * An atmosphere devoid of oxygen, containing gases like methane, ammonia, and water. * Intense volcanic activity and earthquakes. * Constant bombardment by asteroids and meteorites.

3. **Origin of Life (Abiogenesis):** * There is no complete agreement on how this happened, but there are main theories: * **Chemical Origin Theory (or Chemical Synthesis):** Suggests that simple organic molecules (like amino acids and nucleotides) formed from inorganic materials under the conditions of early Earth (such as energy from lightning or ultraviolet radiation). * **Miller-Urey Experiments:** These experiments showed that amino acids could form from inorganic gases using electrical sparks. * **Formation of Complex Molecules:** Simple molecules evolved into complex molecules like proteins and nucleic acids (RNA and DNA). * **Self-Replication:** It is believed that RNA was the primary molecule initially, as it could store information and reproduce. * **Emergence of the First Cells:** Complex molecules evolved to become enclosed by a membrane, leading to the emergence of the first primitive cells (prokaryotic cells).

4. **Oldest Evidence of Life:** * **Microfossils:** The oldest evidence of life is found in rock deposits dating back about 3.5 billion years, resembling ancient bacteria. * **Stromatolites:** These are sedimentary rocks formed from layered bacterial activity, with the oldest dating back about 3.7 billion years.

5. **Evolution of Early Life:** * **Photosynthesis:** Early bacteria evolved to perform photosynthesis, leading to the production of oxygen. * **The Great Oxidation Event:** About 2.4 billion years ago, oxygen began to accumulate in the atmosphere, causing a significant change in Earth's environment. * **Emergence of Eukaryotes:** More complex cells, containing a nucleus and membrane, evolved about 2 billion years ago. * **Sexual Reproduction:** Contributed to increased genetic diversity and accelerated the process of evolution.

6. **Importance of Studying Early Life:** * Understanding our origins. * Understanding the evolution of the planet. * Searching for extraterrestrial life.

**Summary:** Life on Earth began as simple organic molecules that evolved into primitive cells, and then evolved over time to include more complex processes like photosynthesis and the development of eukaryotic cells, paving the way for the immense diversity of life we see today.