Unit Four. The Evolution and Diversity of Life
17. Protists: Advent of the Eukaryotes
You are a eukaryote, an organism composed of cells that contain a nucleus. All the organisms you see around you are eukaryotes, too, as prokaryote organisms are too small for you to see without a microscope to magnify them. Biologists sort the eukaryotes of the living world into four great groups, called kingdoms: animals, plants, fungi, and everything else. This chapter concerns the fourth catch-all group, the protists (kingdom Protista). The beautiful flowerlike creature you see here is a protist, the green algae Acetabularia. It is photosynthetic and grows as long slender stalks as long as your thumb. In the last century some biologists considered it to be a very simple sort of plant. Today, however, most biologists consider Acetabularia to be a protist, restricting the plant kingdom to multicellular terrestrial photosynthetic organisms (and a few marine and aquatic species like water lilies clearly derived from terrestrial ancestors). Acetabularia is marine, not terrestrial, and it is unicellular, with a single nucleus found in the base of its stalk. In this chapter, we will explore how protists are thought to have evolved, and the sorts of creatures found among this most diverse of biological kingdoms. Multicellularity evolved many times within the protists, producing the ancestors of the animal, plant, and fungi kingdoms, as well as several kinds of multicellular algae, some as large as trees.
17.1. Origin of Eukaryotic Cells
The First Eukaryotic Cells
All fossils more than 1.7 billion years old are small, simple cells, similar to the bacteria of today. In rocks about 1.7 billion years old, we begin to see the first microfossils, which are noticeably larger than bacteria and have internal membranes and thicker walls. A new kind of organism had appeared, called a eukaryote (Greek eu, “true,” and karyon, “nut”). One of the main features of a eukaryotic cell is the presence of an internal structure called a nucleus (see section 4.5). As discussed in chapter 15, animals, plants, fungi, and protists are all eukaryotes. In this chapter, we will explore the protists, from which all other eukaryotes evolved. But first we will examine some of the unifying characteristics of eukaryotes, and how they might have originated.
To begin, how might a nucleus have arisen? Many bacteria have infoldings of their outer membranes extending into the interior that serve as passageways between the surface and the cell’s interior. The network of internal membranes in eukaryotes, called the endoplasmic reticulum (ER), is thought to have evolved from such infoldings, as is the nuclear envelope (figure 17.1). The prokaryotic cell shown on the far left has infoldings of the plasma membrane, and the DNA resides in the center of the cell. In ancestral eukaryotic cells, these internal membrane extensions evolved to project farther into the cell, continuing their function as passageways between the interior and exterior of the cell. Eventually, these membranes came to form an enclosure surrounding the DNA, shown on the right, which became the nuclear envelope.
Figure 17.1. Origin of the nucleus and endoplasmic reticulum.
Many bacteria today have infoldings of the plasma membrane. The eukaryotic internal membrane system called the endoplasmic reticulum (ER) and the nuclear envelope may have evolved from such infoldings of the plasma membrane of prokaryotic cells that gave rise to eukaryotic cells.
What was the first eukaryote like? We cannot be sure, but a good model is Pelomyxa palustris, a single-celled, nonphotosynthetic organism that some scientists feel represents an early stage in the evolution of eukaryotic cells. The cells of Pelomyxa are much larger than bacterial cells and contain a complex system of internal membranes. Although they resemble some of the largest early fossil eukaryotes, these cells are unlike those of any other eukaryote: Pelomyxa lacks mitochondria and only rarely undergoes mitosis. However, biologists know very little of the origin of Pelomyxa. It may have lost mitochondria rather than never having had them at all. This primitive eukaryote is so distinctive that it is assigned a phylum all its own, Caryoblastea.
Because of similarities in their DNA, it is widely assumed that the first eukaryotic cells were nonphotosynthetic descendants of archaea.
In addition to an internal system of membranes and a nucleus, eukaryotic cells contain several other distinctive organelles. These organelles were discussed in chapter 4. Two of these organelles, mitochondria and chloroplasts, are especially unique because they resemble bacterial cells and even contain their own DNA. As discussed in section 4.7 and section 15.9, mitochondria and chloroplasts are thought to have arisen by endosymbiosis, where one organism comes to live inside another. The endosymbiotic theory, now widely accepted, suggests that at a critical stage in the evolution of eukaryotic cells, energy-producing aerobic bacteria came to reside symbiotically (that is, cooperatively) within larger early eukaryotic cells, eventually evolving into the cell organelles we now know as mitochondria. Similarly, photosynthetic bacteria came to live within some of these early eukaryotic cells, leading to the evolution of chloroplasts (figure 17.2), the photosynthetic organelles of plants and algae. Now, let’s examine the evidence supporting the endosymbiotic theory a little more closely.
Figure 17.2. The theory of endosymbiosis.
Scientists propose that ancestral eukaryotic cells engulfed aerobic bacteria, which then became mitochondria in the eukaryotic cell. Chloroplasts may also have originated in this way, with eukaryotic cells engulfing photosynthetic bacteria that became chloroplasts.
Mitochondria. Mitochondria, the energy-generating organelles in eukaryotic cells, are sausage-shaped organelles about 1 to 3 micrometers long, about the same size as most bacteria. Mitochondria are bounded by two membranes. The outer membrane is smooth and was apparently derived from the host cell as it wrapped around the bacterium. The inner membrane is folded into numerous layers, embedded within which are the proteins of oxidative metabolism.
During the billion-and-a-half years in which mitochondria have existed as endosymbionts within eukaryotic cells, most of their genes have been transferred to the chromosomes of the host cells—but not all. Each mitochondrion still has its own genome, a circular, closed molecule of DNA similar to that found in bacteria, on which is located genes encoding some of the essential proteins of oxidative metabolism. These genes are transcribed within the mitochondrion, using mitochondrial ribosomes that are smaller than those of eukaryotic cells, very much like bacterial ribosomes in size and structure. Mitochondria divide by simple fission, just as bacteria do, and can divide on their own without the cell nucleus dividing. Mitochondria also replicate and sort their DNA much as bacteria do. However, the cell’s nuclear genes direct the process, and mitochondria cannot be grown outside of the eukaryotic cell, in cell-free culture.
Chloroplasts. Many eukaryotic cells contain other endosymbi- otic bacteria in addition to mitochondria. Plants and algae contain chloroplasts, bacteria-like organelles that were apparently derived from symbiotic photosynthetic bacteria. Chloroplasts have a complex system of inner membranes and a circle of DNA. While all mitochondria are thought to have arisen from a single symbiotic event, it is difficult to be sure with chloroplasts. Three biochemically distinct classes of chloroplasts exist, but all appear to have their origin in the cyanobacteria.
Red algae and green algae seem to have acquired cyanobacteria directly as endosymbionts, and may be sister groups. Other algae have chloroplasts of secondary origin, having taken up one of these algae in their past. The chloroplasts of euglenoids are thought to be green algal in origin, while those of brown algae and diatoms are likely of red algal origin. The chloroplasts of dinoflagellates seem to be of complex origins, which might include diatoms.
As mentioned earlier, the primitive eukaryote Pelomyxa does not exhibit mitosis, the eukaryotic process of cell division. How did mitosis evolve? The mechanism of mitosis, now so common among eukaryotes, did not evolve all at once. Traces of very different, and possibly intermediate, mechanisms survive today in some of the eukaryotes. In fungi and some groups of protists, for example, the nuclear membrane does not dissolve, and mitosis is confined to the nucleus. When mitosis is complete in these organisms, the nucleus divides into two daughter nuclei, and only then does the rest of the cell divide. This separate nuclear division phase of mitosis does not occur in most protists, or in plants or animals. We do not know if it represents an intermediate step on the evolutionary journey to the form of mitosis that is characteristic of most eukaryotes today, or if it is simply a different way of solving the same problem. There are no fossils in which we can see the interiors of dividing cells well enough to be able to trace the history of mitosis.
Key Learning Outcome 17.1. The theory of endosymbiosis proposes that mitochondria originated as symbiotic aerobic bacteria and chloroplasts originated from a second endosymbiotic event with photosynthetic bacteria.