Evidence comes from fossils of single-celled prokaryotes found in
rocks that are more than 3 billion years old. At that time, there was
little oxygen in the atmosphere. The earliest life was therefore
anaerobic (Latin for “without oxygen”). Anaerobic bacteria do not
require oxygen for cellular respiration. Today, anaerobic bacteria
thrive in places that have little or no oxygen, like swamps.
Bacteria
increased oxygen
in Earth’s
atmosphere
Over time, some bacteria developed the ability to use
photosynthesis. Cyanobacteria, still in existence today, were
one of the first photosynthetic bacteria. One of the products of
photosynthesis is oxygen. Over hundreds of millions of years, the
amount of oxygen in Earth’s atmosphere increased. This allowed
aerobic bacteria to develop. Aerobic bacteria use oxygen for
cellular respiration. There are many different species of aerobic
bacteria living today.
Eukaryotic cells
developed from
prokaryotic cells
Eventually, eukaryotic cells developed from bacteria. A scientific
theory states that long ago, smaller prokaryotic cells were engulfed
by larger prokaryotic cells. The smaller cells began to survive by
living inside of the larger cells. Over time they took on specific
functions inside the larger cells like producing energy. Eventually,
the smaller cells became the organelles (like mitochondria) inside
of eukaryotic cells (Figure 9.9).