Question

The origin of life on Earth most likely occurred (a) before 4.5 billion years ago; (b) between about 4.5 and 3.5 billion years ago; (c) between about 3.0 and 2.5 billion years ago.

Short answer

Life most likely originated between 4.5 and 3.5 billion years ago.

Step-by-step solution

Understanding the Question

The question asks about the timeline of the origin of life on Earth, providing three different time ranges to choose from. We need to determine which time range is most likely based on scientific understanding.

Eliminating Improbable Options

The first option, before 4.5 billion years ago, is unlikely because Earth itself formed approximately 4.5 billion years ago, making the existence of life before this point improbable.

Considering Geological Evidence

The most widely supported scientific evidence suggests that life began during the Archean Eon. Geological findings and fossil records, such as stromatolites, date back to around 3.7 billion years ago, which aligns with the second given time range.

Evaluating the Second Option

Option (b), which states life originated between about 4.5 and 3.5 billion years ago, corresponds with the earliest evidence of life and the formation of the earliest continental crusts where life could have emerged.

Rejecting the Later Time Frame

Option (c), between 3.0 and 2.5 billion years ago, is too late considering the existing evidence, which shows life had already been established by this period.

Key concepts

Archean Eon

The Archean Eon is a fascinating period in Earth's history, spanning from approximately 4 to 2.5 billion years ago. This era is particularly significant when considering the origins of life on Earth. During the Archean Eon, the planet underwent dramatic changes that supported the emergence of life.

Earth's crust began to stabilize during this time, forming the first continental blocks. These proto-continents provided a stable platform for complex chemical reactions essential for life. Moreover,

• the atmosphere was primarily composed of nitrogen and carbon dioxide, but lacked free oxygen,
• early life likely thrived in extreme environments such as hot springs and deep-sea hydrothermal vents.

Despite these harsh conditions, life forms such as prokaryotic cells, including bacteria and archaea, began to appear. This period laid crucial groundwork for life to evolve and diversify.

Geological Evidence

Geological evidence is key to understanding the early beginnings of life on Earth. Scientists rely on various types of geological records to piece together this ancient history. One such vital clue is the presence of stromatolites.

Stromatolites are layered formations created by cyanobacteria, a type of photosynthesizing microorganism. These structures are found in ancient rocks dated to about 3.5 billion years ago, providing direct evidence of life during the Archean Eon. In addition to stromatolites, other significant geological markers include:

• banded iron formations, indicating early photosynthetic activity,
• isotopic signatures suggesting biological carbon fixation,"
• and microfossils, which are extremely small remnants of early living organisms.

These pieces of evidence are crucial in constructing a timeline for the emergence of life, supporting the theory that life began over 3.5 billion years ago.

Fossil Records

Fossil records from the Archean Eon provide compelling insights into the early life forms that inhabited the Earth. Despite the challenges of studying fossils from this ancient period—due to their often microscopic size or poorly preserved state—they are invaluable in our understanding of life's early history.

The discoveries of microfossils, which are fossilized forms of bacteria, offer direct glimpses into the past. These microfossils are found in ancient rocks and are typically similar to modern bacteria, suggesting primitive life forms were well-established during the Archean Eon.

• The earliest known fossils appear as minute filaments and spherical forms dated to about 3.5 billion years ago.
• These simple cellular structures provide clues about the environmental conditions of early Earth.
• They are also instrumental in demonstrating the existence of life during this tumultuous time.

Overall, fossil records serve as critical evidence, showing that life's complexity began from simple beginnings, flourishing over billions of years.