4.1 Reading Resource #2: The First Cellular Organism Several additional steps had to occur before a true living cell could emerge from macromolecular
aggregations. Scientists have coined the acronym LUCA to refer to the hypothesized last universal common
ancestor for discussion purposes.
Microfossils , which are ancient remains of microscopic life, indicate that the first cells were
prokaryotic.
Stromatolites , a type of early cell fossil, are rocklike columns made up of many minute layers of
prokaryotic cells, or microbial biofilms. Sediment accumulates and mineralizes around the cells over time. In
the meantime, a new layer of living cells grows on top of the older, dead cells.
The organic soup hypothesis contends that the earliest cells did not synthesize the organic molecules
they required, but instead obtained them from the environment. Many types of organic molecules, such as
sugars, nucleotides, and amino acids, may have been consumed by these primitive
heterotrophs .
Eukaryotes may have appeared in the fossil record as early as 2.2 billion years ago, and geochemical
evidence suggests that they were present much earlier. Prokaryotes gave rise to eukaryotes. Nuclear envelopes,
as well as other membranous organelles like mitochondria and chloroplasts, are absent in archaeal and bacterial
cells. How did these organelles form? According to the serial endosymbiosis hypothesis, organelles like
mitochondria and chloroplasts could have evolved because of mutually beneficial symbiotic relationships
between two prokaryotic organisms. Chloroplasts are thought to have evolved from photosynthetic bacteria
(cyanobacteria) within larger heterotrophic cells, whereas mitochondria are thought to have evolved from
aerobic bacteria (possibly ancient purple bacteria) within larger anaerobic cells.
4.1 Reading Resource #3
