The following passage is adapted from an essay about evolutionary biology.
For much of the twentieth century, evolutionary biology adhered strictly to the modern synthesis, a framework that viewed genetic mutation and natural selection within individual lineages as the sole drivers of evolutionary change. In 1967, however, a young biologist named Lynn Sagan (later Lynn Margulis) challenged this orthodox view. She proposed the endosymbiotic theory, which argued that key eukaryotic organelles—specifically mitochondria and chloroplasts—originated as free-living prokaryotic bacteria that were engulfed by ancestral host cells. Instead of evolving through gradual point mutations, complex cells arose through symbiogenesis: the merging of separate organisms into a single, cooperative unit.
Initially, Margulis’s hypothesis was met with intense skepticism and outright rejection by the scientific establishment. Critics argued that natural selection could not favor such a radical union and that there was no physical mechanism to explain how engulfed bacteria could survive within a host. To build her case, Margulis systematically compiled various lines of evidence. She pointed out that mitochondria and chloroplasts are roughly the same size as bacteria and divide independently of the host cell through binary fission, a process identical to bacterial reproduction.
The turning point for the theory came in the late 1970s and 1980s with the advent of molecular sequencing technology. Researchers discovered that mitochondria and chloroplasts contain their own distinct genomes, which are circular, DNA-based loops devoid of histones, matching the structure of prokaryotic DNA rather than the linear DNA found in the eukaryotic nucleus. Furthermore, RNA sequencing of these organelles revealed that their ribosomal RNA (rRNA) was far more closely related to specific groups of modern bacteria—specifically alphaproteobacteria for mitochondria and cyanobacteria for chloroplasts—than to the cytoplasm of the host eukaryotic cells.
Today, endosymbiosis is no longer a fringe hypothesis but a cornerstone of evolutionary biology. The story of its acceptance illustrates how scientific progress often depends on the integration of morphological observations with molecular evidence, eventually forcing a paradigm shift in how we understand the tree of life.
Based on the passage, what is the correct chronological sequence of the scientific developments and arguments described?
- 1Biologists rely exclusively on natural selection within individual lineages to explain evolutionary change.
- 2The scientific community rejects the endosymbiotic hypothesis due to a perceived lack of physical mechanisms for host cell survival.
- 3Margulis compiles evidence based on organelle size and replication methods to defend her theory.
- 4Molecular sequencing reveals circular genomes within mitochondria and chloroplasts.