How did eukaryotes evolve from prokaryotes?

Evolution of Eukaryotes from Prokaryotes

How did Eukaryotes Evolve from Prokaryotes? Long Question Note – Class 12 Biology (Chapter 24 Evolution) (7–8 Marks – RTS/HED/Punjab Board Pattern)

Introduction

Prokaryotes (bacteria & archaea) are simple cells without a nucleus or membrane-bound organelles. Eukaryotes (plants, animals, fungi, protists) have a true nucleus and organelles like mitochondria, chloroplasts, ER, Golgi, etc.

The transition from prokaryotes to eukaryotes is one of the major events in the history of life. Two main scientific hypotheses explain this evolutionary step:

1. Membrane Invagination Hypothesis (Infolding Theory)

Proposed as an alternative to symbiosis.

Mechanism (Step-by-step):

A primitive prokaryotic cell (probably an archaeon) had a highly folded plasma membrane.
These folds (invaginations) pinched off and formed internal membrane compartments.
One large invagination enclosed the genetic material → formed the nuclear membrane and nucleoplasm.
Further invaginations gave rise to endoplasmic reticulum (ER), Golgi apparatus, and vesicles.
Mitochondria-like structures may have formed by further folding and specialization.

Evidence:

Nuclear membrane and ER are continuous with each other.
No foreign DNA in organelles (unlike endosymbiont theory).

Diagram – Membrane Invagination Hypothesis

2. Endosymbiont Hypothesis (Most Accepted)

Proposed by Lynn Margulis (1967).

Mechanism:

A primitive anaerobic eukaryotic-like host cell (probably an archaeon) engulfed free-living aerobic bacteria (proto-mitochondria) by endocytosis.
Instead of digesting them, the bacteria lived inside in mutual symbiosis.
Later, the host engulfed photosynthetic cyanobacteria (proto-chloroplasts).
Over millions of years, these symbionts lost many genes and became permanent organelles (mitochondria & chloroplasts).

Evidence (Very Important for Exams):

Mitochondria and chloroplasts have their own circular DNA (like bacteria).
They have 70S ribosomes (prokaryotic type).
They divide by binary fission independently of the host cell.
Double membrane: outer from host, inner from symbiont.
Sequence similarity between mitochondrial DNA and α-proteobacteria, chloroplast DNA and cyanobacteria.

Diagram – Endosymbiotic Hypothesis