Question

Early life arose in an oxygen-free environment, but if any of these microbes had somehow come in contact with oxygen, the most likely effect would have been (a) nothing at all; (b) to increase their metabolic rates; (c) to kill them.

Short answer

(c) to kill them.

Step-by-step solution

Understand the Context of Early Life

Early life forms on Earth evolved in an environment without oxygen. They were anaerobic organisms, meaning they did not rely on oxygen for their metabolic processes.

Analyze the Impact of Oxygen on Anaerobic Organisms

When anaerobic organisms, which thrive in oxygen-free environments, are exposed to oxygen, especially those not adapted to it, oxygen can be toxic due to its reactive nature. This can cause oxidative damage to cellular components.

Consider the Likely Effects of Oxygen

Since these early microbes had evolved in environments devoid of oxygen, sudden exposure without any evolved protective mechanisms would likely lead to harmful effects, potentially killing the microbes.

Key concepts

Anaerobic Organisms

Anaerobic organisms are fascinating life forms that thrive without oxygen. In the early days of Earth's history, these organisms were the primary life forms, residing in environments completely absent of oxygen. Unlike aerobic organisms, anaerobes do not use oxygen to fuel their metabolic processes. Instead, they rely on other methods like fermentation to obtain energy.

There are two main types of anaerobic organisms:

• Obligate anaerobes: Completely unable to tolerate oxygen, often poisoned by its presence.
• Facultative anaerobes: Capable of surviving without oxygen, but can also use oxygen when it is available.

These organisms are important for understanding early life and its adaptation strategies. Their simple yet effective energy-processing mechanisms allowed them to survive and thrive in primitive conditions.

Oxygen-Free Environment

Oxygen-free environments, referred to as anoxic, were prevalent on the early Earth. The atmosphere was drastically different from today, with gases like methane, ammonia, and hydrogen being more common. This lack of oxygen provided a distinct environment where anaerobic organisms could flourish without competition from oxygen-breathing life forms.

These anoxic conditions not only shaped the metabolic strategies of these early life forms, allowing them to develop unique processes, but also influenced the evolutionary pathways that these organisms would take over time. Such environments are still prevalent today in places like deep-sea hydrothermal vents, swamps, and inside the guts of some animals, serving as a reminder of early Earth’s conditions.

Impact of Oxygen on Life

The introduction of oxygen into Earth's environment had a significant impact on life. For the primarily anaerobic early life forms, oxygen was a new and potent threat. This is because oxygen is highly reactive and can cause damage to cellular components through a process known as oxidative stress.

Without any protections against oxygen's reactive properties, many anaerobic organisms faced existential threats. This led to selective pressures that eventually gave rise to species capable of surviving, and later thriving, with oxygen. As a result, new metabolic pathways, such as the use of oxygen for aerobic respiration, evolved. This not only allowed organisms to harness more energy from their environment but also paved the way for more complex life forms to develop, eventually leading to the diversity of life we see today.