Chapter 1
Chapter 1
On the Origin of Mitosing Cells LYNN SAGAN Department of Biology, Boston University Boston, Massachusetts, U.S.A. (Received eight June nineteen sixty-six)
A theory of the origin of eukaryotic cells ("higher" cells which divide by classical mitosis) is presented. By hypothesis, three fundamental organelles : the mitochondria, the photosynthetic plastids and the nine plus two basal bodies of flagella were themselves once free-living (prokaryotic) cells. The evolution of photosynthesis under the anaerobic conditions of the early atmosphere to form anaerobic bacteria, photosynthetic bacteria and eventually blue-green algae (and protoplastids) is described. The subsequent evolution of aerobic metabolism in prokaryotes to form aerobic bacteria (protoflagella and protomitochondria) presumably occurred during the transition to the oxidizing atmosphere. Classical mitosis evolved in protozoan-type cells millions of years after the evolution of photosynthesis. A plausible scheme for the origin of classical mitosis in primitive amoeboflagellates is presented. During the course of the evolution of mitosis, photosynthetic plastids (themselves derived from prokaryotes) were symbiotically acquired by some of these protozoans to form the eukaryotic algae and the green plants.
The cytological, biochemical and paleontological evidence for this theory is presented, along with suggestions for further possible experimental verification. The implications of this scheme for the systematics of the lower organisms is discussed.
One. Introduction
One. Introduction
All free-living organisms are cells or are made of cells. There are two basic cell types: prokaryotic and eukaryotic. Prokaryotic cells include the eubacteria, the blue-green algae, the gliding bacteria, the budding bacteria, the pleuropneumonia-like organisms, the spirochaetes and rickettsias, etc. Eukaryotic cells, of course, are the familiar components of plants and animals, molds and protozoans, and all other "higher" organisms. They contain subcellular organelles such as mitochondria and membrane-bounded nuclei and have many other features in common.
"The numerous and fundamental differences between the eukaryotic and prokaryotic cell which have been described in this chapter have been fully recognized only in the past few years. In fact, this basic divergence in cellular structure which separates the bacteria and blue-green algae T.B. 225 16
from all other cellular organisms, probably represents the greatest single evolutionary discontinuity to be found in the present-day living world" (Stanier, Douderoff & Adelberg, 1963).
This paper presents a theory of the origin of this discontinuity between eukaryotic (mitosing or "higher") and prokaryotic cells. Specifically, the mitochondria, the nine plus two basal bodies of the flagella, and the photosynthetic plastids can all be considered to have derived from free-living cells, and the eukaryotic cell is the result of the evolution of ancient symbioses. Although these ideas are not new, in this paper they have been synthesized in such a way as to be consistent with recent data on the biochemistry and cytology of subcellular organelles. In accord with both the fossil record and this theory, the lower eukaryotes (protozoans, eukaryotic algae and fungi) can now be included on a single phylogenetic tree. In contrast to previous thought on the subject, many aspects of this theory are verifiable by modern techniques of molecular biology.
In defending the idea that the eukaryotic cell arose by a specific series of endosymbioses, a plausible scheme for the origin of mitosis itself emerges. ("Mitosis" is meant only in the classical sense; the analogous equal distribution of genes to daughter cells in prokaryotes is not relevant here.)
The paper is organized into three parts. The first presents the theory of the origin of the eukaryotic cell. The second part is a compilation of the scientific literature in support of the sequences presented first. The last section suggests some experimental results which can be predicted from the theory.