Question

Which of the following is not considered a key piece of evidence supporting a common ancestor for all life on Earth? (a) the fact that all life on Earth is carbon-based; (b) the fact that all life on Earth uses the molecule ATP to store and release energy; (c) the fact that all life on Earth builds proteins from the same set of left-handed amino acids.

Short answer

Option (a) is not considered evidence of a common ancestor for all life.

Step-by-step solution

Understand the Common Ancestry Concept

A common ancestor implies that all organisms descended from a single origin or entity. This means there are fundamental biochemical and genetic similarities shared by all life forms on Earth.

Analyze the Evidence

Look at the provided options and evaluate their significance in suggesting all life originated from a common ancestor. - Option (a) refers to all life being carbon-based. - Option (b) refers to the use of ATP for energy transactions. - Option (c) refers to using left-handed amino acids in proteins.

Evaluate Option (a)

Although being carbon-based is a common feature of life, it is a requirement for any organic molecule and is not unique enough to serve as evidence for common ancestry. Different forms of life on a different planet could also be carbon-based without sharing a common ancestor with Earth's life forms.

Evaluate Option (b)

The use of ATP across diverse life forms suggests a common evolutionary origin, as ATP serves a specific biochemical function that is consistently utilized in similar ways across all known life.

Evaluate Option (c)

Using the same set of left-handed amino acids is a striking similarity among all terrestrial life, implying a shared origin since this choice is arbitrary and specific among the possible biochemical alternatives.

Determine the Unrelated Evidence

From the analysis, the unique utilization of ATP and specific amino acid chirality strongly indicate a shared ancestry. Unlike these, the carbon-based aspect is a general characteristic not directly connected to evidence of common descent on its own.

Key concepts

Carbon-Based Life

When we explore the idea of carbon-based life, we are diving into the fundamental structure of living organisms on Earth. Carbon is a versatile element that can form stable bonds with many elements, including itself. This capacity enables the creation of complex molecules that are essential for life. Carbon molecules form the backbone of biological molecules like proteins, carbohydrates, and lipids.
This universality of carbon in Earth's organisms arises because carbon can form long chains and rings, creating a vast variety of chemical compounds.
However, carbon-based life is not unique to Earth. Theoretically, other life forms elsewhere in the universe could also be carbon-based without sharing a common ancestry with Earthling life forms. This is because carbon's chemical properties make it favorable for life's chemistry. Thus, while being carbon-based is widespread among life on Earth, it does not specifically serve as evidence for common ancestry. Instead, it is a general feature of organic chemistry.

ATP Energy Molecule

The ATP energy molecule, short for adenosine triphosphate, is considered an essential energy currency for cells. All known life forms utilize ATP to store and release energy, underscoring its critical role in cellular processes.
ATP performs its role through its phosphate groups. When a phosphate group is removed from ATP, energy is released, which cells use to carry out vital processes, such as muscle contraction, nerve impulse propagation, and chemical synthesis.
The ubiquitous use of ATP in vastly different organisms suggests it evolved early in the history of life and has been conserved due to its efficiency and effectiveness.

• The consistent use of ATP across all life forms is strong evidence for a common evolutionary origin.
• The fact that such a specific molecule is used universally implies that life on Earth shares a common ancestry and that ATP's function was present in our last universal common ancestor.

Left-Handed Amino Acids

Amino acids are the building blocks of proteins, and they come in two forms: left-handed (L-form) and right-handed (D-form). However, all proteins in living organisms on Earth are made from left-handed amino acids. This uniformity is peculiar and significant.
The preference for left-handed amino acids is a curious feature, pointing towards a common origin. Theoretically, life could have adopted proteins made from right-handed variants, but it hasn't.
The fact that life chose one specific form of amino acids is strong evidence of a shared ancestry because this decision appears arbitrary yet universal.
This choice of chirality (handedness) is arbitrary but consistent across all terrestrial life. It serves as convincing evidence of a shared origin:

• The uniformity in using only left-handed amino acids suggests a common starting point due to chance in the early conditions where life began.
• Since this is a unique identifier and not required by the chemistry itself, it strongly supports the idea of a common ancestor for all living beings on Earth.