The early cells capable of ingesting and domesticating other cells were larger and more flexible than their prokaryotic neighbors. They had distinct nuclei, mito- chondria, and other internal organelles. These were eukaryotes: the ancestors of all animals, plants, fungi, and countless kinds of single-celled organisms like
Figure 2.2 Lichens are a symbiotic association between a fungus and algal cells. The algae (spherical cells) provide photosynthetic product to the fungus, which provides water and minerals, as well as protection, for the algae. Drawing from Brown 1935.
amoebas, paramecia, and algae. These larger, more complex cells began to appear probably around 2.5 billion years ago. They were the first hunters, and cyanobac- teria were probably their most abundant prey.
We know little about the nature of those protoeukaryotic cells or exactly how they began to consume other cells. They were evidently descended from an archaean, a prokaryote similar to true bacteria in size, shape, and simplic- ity of structure, but differing in many details of their cell walls, chromosomes and metabolism. The ancestral eukaryote appears to be most closely related to modern archaeans called thermoacidophiles, organisms that thrive in water of extreme temperatures or acidity.
These protoeukaryotes were remarkable in several ways and quite unlike other prokaryotes. They were flexible, “naked” cells without walls, and capable to some degree of changing shape. They at least could form pockets at their surfaces that, like tiny mouths, could pinch off around food items and bring them inside. They were early protoypes of modern amoebas (Fig. 2.3), single-celled predators that move and feed in this shape-shifting way.
The flexibility required for this mode of feeding was made possible by a most remarkable innovation: the cytoskeleton—a complex, dynamic system of protein rods, tubules, and small mobile units that could move things around within the cell. The cytoskeleton, as we know it today, is as much a muscular system as it is a skele- ton. It enables cells like amoebas to change their shapes or move forward by shifting cytoplasm from one part of the cell to another. This results in the familiar creeping movement that may have been the inspiration for the 1950s horror classic The Blob.
Equally important is the ability to move things around inside the cell. Small packages, or vesicles, containing food, waste products, or other materials are carried by tiny motor proteins that “walk” along tracks within the cytoskeletal matrix. Chromosomes are moved around in a similar way during cell division. The ability to move things around inside allows eukaryotic cells to get much larger than prokaryotes, which rely on diffusion for internal distribution. So somewhere in the mid-2-billions, large sophisticated cells had gotten into the business of
Figure 2.3 Amoeboid cells change shape by extending portions of the internal cytoskeleton. This allows the cell to move in a particular direction as well as to form
eating other cells. It is not hard to imagine then, that, like sea slugs, some of them domesticated cyanobacteria to become photosynthetic.
Endosymbiois
In the early 1970s a young biologist named Lynn Margulis achieved notoriety for reviving the “outrageous” theory that not only chloroplasts, but also mito- chondria (aerobic energy-processing organelles) were originally free-living bac- teria that came to dwell inside early eukaryotic cells (e.g., Margulis 1998). This scenario, first proposed by the Russian biologist Mereschkowski in 1905, had been derided, discarded, and largely forgotten by the scientific community until revived by Margulis and others of her generation.
Both mitochondria and chloroplasts retain vestiges of their former indepen- dent life, including their own bacterial loops of DNA and interior membranes. A few algae (glaucocystophytes) even retain vestiges of peptidoglycan, the unique wall material of bacteria, around their chloroplasts. The incorporation of whole cells, in this case ancient bacteria, into larger cells is called endosymbiosis. The endosymbiosis of ancient bacteria into larger cells, followed by their transforma- tion into mitochondria and chloroplasts (Fig. 2.4), is now firmly accepted by the
Ancient prokaryote
Proto-eukaryote with flexible surface
Food items Evolving endoplasmic reticulum Symbiotic or prey bacterium Enclosed symbiont (mitochondrion or chloroplast) Double nuclear membrance
Figure 2.4 The evolution of the cytoskeleton and loss of the cell wall allowed the first eukaryotic cells to become larger and more flexible. Portions of the cell membrane extended inward to form vacuoles, endoplasmic reticulum, the nuclear envelope, and other internal organelles.
scientific community and presented as a matter of fact in freshman biology text- books. Endosymbiosis leading to chloroplasts probably happened only once (pos- sibly twice), while kleptoplasty, the theft of those chloroplasts from one eukaryote by another, happened many times.
Protists
The earliest eukaryotes, and numerous of their simple modern descendants, are by definition protists (also called protoctists). They are mostly aquatic organisms ranging from single-celled amoebas to multicellular algae (“seaweeds”). They represent a level of organization that is simpler than that of plants, animals, and fungi, and for many years were classified together as the kingdom Protista. Large seaweeds, such as the giant kelps, have three-dimensional tissues and distinct stem- and leaflike organs, and so are very much like the higher plants that live on the land. It is therefore hard to define precisely what a protist is, other than “eukaryotes that are not plants, animals, or fungi.” True plants have a greater spe- cialization of cells, tissues, and organs, and protect their young embryos within special chambers, something not found in any protist.
Modern phylogenetic analysis has revealed that the protists represent a wide and varied assortment of independent eukaryotic lineages. Animallike, plantlike, and fungus-like traits have evolved multiple times among them, resulting in unre- lated organisms that happen to look or behave like each other (convergent evolu- tion). The word “alga” refers to any of the great variety of photosynthetic protists, including multicellular seaweeds. Terrestrial plants, animals, and fungi—what we consider three of the major kingdoms of life—are relatively recent branches of some of these ancient protist lineages and are well defined. So the word protist represents a level of complexity, not a distinct taxonomic group.
Protists are often highly mobile, particularly those that have specialized as ani- mallike predators (“protozoa”). The first protists were probably similar to amoe- bas, organisms that literally “surround” their food. Others evolved fixed, feeding pockets that funnel food down into food vacuoles. Paramecium is one of the most successful and widespread of these single-celled predators. Others, like Vorticella or Diplodinium, attach to rocks or underwater plants and just wait for food to come by, which they catch with tentacles surrounding their mouths.
Many protists move about by means of flagella, as do some bacteria, but with flagella of radically different structure and operation. Unlike the protein fila- ments that spin like propellers in bacteria, flagella in protists evolved from narrow extension of the cell itself, complete with cell membrane and cytoskeleton ele- ments. These flagella contain a distinctive arrangement of long protein microtu- bules, consisting of a ring of double tubules and two single tubules in the center (Fig. 2.5). Such flagella are whiplike, waving back and forth as the microtubules
on either side alternately contract. While flagella are obviously of value to aggres- sive predatory cells, many single-celled algae also have them, as do reproductive cells of many multicellular seaweeds.
Lynn Margulis provided another idea, one that has not been so popular. According to her (see Margulis 1998), a bacterium with multiple simple flagella, something like a modern spirochaete, attached itself to an ancient eukaryotic cell, lost much of its own cellular structure, and evolved into the eukaryotic flagellum. In this model, the spirochaete contributed to the evolution of the cytoskeleton by partially moving inside the cell. Some modern protists, such as the Mixotricha paradoxa that live in the guts of termites, do in fact have spirochaetes attached to their cell surface, and they may help them move around. The idea that eukaryotic flagella evolved from such a symbiosis has always been highly controversial, and there remains little hard evidence for it.
Eukaryotes did not replace prokaryotes, but instead created vast new habitats for them. Billions of individual bacteria, representing hundreds of species, for example, inhabit each human body. But these are largely invisible. The world as we perceive it is made up of the much larger eukaryotes.