Eukaryotic Origins*
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Eukaryotes
Living things fall into three large groups:
The earliest fossils found appear to be bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for bacteria, relatively large cells. Most other bacteria and archaea have small cells, 1 or 2
Characteristics of eukaryotes
Data from these fossils have led biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor, because these characteristics are present in at least
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores. This is the single characteristic that is both necessary and sufficient to define an organism as
a eukaryote . All extant eukaryotes have cells with nuclei. - Mitochondria. Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria.
- A cytoskeleton containing the structural and motility components called
actin microfilaments and microtubules. All extant eukaryotes have these cytoskeletal elements. - Flagella and cilia, organelles associated with cell motility. Some extant eukaryotes lack flagella and/or cilia, but
they are descended from ancestors that possessed them. - Chromosomes, each
consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histonesclearly evolved from ancestors that had them. - Mitosis, a process of nuclear division wherein replicated chromosomes
are divided and separated using elements of the cytoskeleton. Mitosis is universally present in eukaryotes. - Sex, a process of genetic recombination unique to eukaryotes in which diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid nuclei and subsequent karyogamy, a stage where two haploid nuclei fuse
together to create a diploid zygote nucleus. - Members of all major lineages have cell walls, and it might be reasonable to conclude that the last common ancestor could make cell walls during some stage of its life cycle. However, not enough
is known about eukaryotes’ cell walls and their development to know how much homology exists among them. If the last common ancestor could make cell walls,it is clear that this ability must have been lost in many groups.
Endosymbiosis and the evolution of eukaryotes
Bacterial and archaeal metabolism
Many important metabolic processes arose in bacteria and archaea, and some of these, such as nitrogen fixation,
While today’s atmosphere is about one-fifth molecular oxygen (O2), geological evidence shows that it originally lacked O2. Without oxygen, aerobic respiration would not
Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds. Various metabolic processes evolved that protected organisms from oxygen; one of which, aerobic respiration, also generated high levels of ATP. It became widely present among microbes, including in a group we now call alpha-
Recall that the first fossils that we believe to be eukaryotes are about 2 billion years old, so they appeared as oxygen levels were increasing. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. These organelles were first observed by light microscopists in the late 1800s, where they appeared to be worm-shaped structures that seemed to move around in the cell. Some early observers suggested that they might be bacteria living inside host cells, but these hypotheses remained unknown or rejected in most scientific communities.
Endosymbiotic theory
As cell biology developed in the twentieth century,
Broadly, it has become clear that many of our nuclear genes and the molecular machinery responsible for replication and expression appear closely related to those in
Mitochondria
One of the major features distinguishing bacteria and archaea from eukaryotes is mitochondria. Eukaryotic cells may contain anywhere from one to several thousand mitochondria, depending on the cell’s level of energy consumption. Each mitochondrion measures 1 to 10 or greater micrometers