Explain how the process of natural selection could have acted on protocells to give rise to the first true cells.

IB DP Biology Higher Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Protocells are lipid-vesicle structures that can encapsulate molecules, grow, and divide spontaneously. Some researchers propose that variation among protocell populations allowed natural selection to operate before the emergence of true cells.

Model answer (4 marks)

1. Protocells could encapsulate different molecules, such as RNA or catalytic molecules, giving rise to variation in growth or reproduction.
2. Protocells whose contents enhanced membrane growth, stability or division would have a selective advantage and be more likely to persist.
3. The advantageous molecules would be inherited by daughter vesicles during division, providing a form of heredity.
4. Over many generations, selection would accumulate beneficial molecules (e.g. catalytic RNA, later DNA), driving the transition to true cells with reliable replication and metabolism.

Examiner tips

Use the word "variation" and "heritable" to show understanding of selection. Mention that protocells with better growth/division survive. Explain inheritance through division. Show that accumulation of benefits leads to true cells.

Common mistakes

Confusing protocells with true cells; not linking selection to heredity. Using vague terms like "better" without specifying membrane growth or stability. Omitting the role of catalytic molecules in enhancing protocell fitness.

Mark scheme (4 marks)

Protocells encapsulating different molecules (e.g. RNA or catalytic molecules) would show variation in their ability to grow or reproduce, providing heritable variation within the population.
Protocells whose encapsulated molecules (e.g. ribozymes) enhanced membrane growth, stability, or division would have a selective advantage and be more likely to persist.
The advantageous molecules (and hence the protocell's properties) could be passed to daughter vesicles upon division, providing a form of heredity.
Over many generations, selection would accumulate beneficial molecules (such as catalytic RNA and eventually DNA-based genomes), driving the transition from simple protocells to true cells with more reliable replication and metabolism.

Key terms in this question

protocell · natural selection