CONCEPTS IN BIOLOGY

PART V. THE ORIGIN AND CLASSIFICATION OF LIFE

 

19. The Origin of Life and the Evolution of Cells

 

19.5. Major Evolutionary Changes in Early Cellular Life

Once living things had genetic material that stored information and could mutate, they could evolve. Thus, living things could have proliferated into a variety of kinds that were adapted to specific environmental conditions. Remember that Earth has not been static but has been changing as a result of its cooling, volcanic activity, and encounters with asteroids. In addition, the presence of living organisms has had an impact on the way in which Earth has developed. Regardless of the way in which life originated on Earth, there have been several major events in the subsequent evolution of living things.

The Development of an Oxidizing Atmosphere

Since its formation, Earth has undergone constant change. In the beginning, it was too hot to support an atmosphere. Later, as it cooled and as gases escaped from volcanoes, a reducing atmosphere (one lacking oxygen) was likely to have been formed. The earliest life-forms would have lived with a reducing atmosphere. However, now we have an oxidizing atmosphere which contains 20 percent oxygen. Today, most organisms use the oxygen to extract energy from organic molecules through a process of aerobic respiration. But what caused the atmosphere to change? It is clear that the oxygen in our current atmosphere is the result of the process of photosynthesis.

The Origin of Photosynthesis

Today, we find that several kinds of Bacteria perform some form of photosynthesis in which sunlight is used to synthesize organic molecules from inorganic molecules. Several of these perform a type of photosynthesis that does not result in the release of oxygen. However, one major group, the cyanobacteria, use a form of photosynthesis that results in the release of oxygen. Therefore, it seems plausible that the first organisms, regardless of whether they were heterotrophs or chemo- autotrophs, accumulated many mutations over time, resulting in photosynthetic autotrophs. Because oxygen is released from the most common form of photosynthesis, this would have resulted in the development of an oxidizing atmosphere.

The development of an oxidizing atmosphere created an environment unsuitable for the continued spontaneous formation of organic molecules. Organic molecules tend to break down (oxidize) when oxygen is present. The presence of oxygen in the atmosphere would make it impossible for life to spontaneously originate in the manner described earlier in this chapter because an oxidizing atmosphere would not allow the accumulation of organic molecules in the seas. However, once living things existed, new life could be generated through reproduction, and new kinds of life could be generated through mutation and evolution. The presence of oxygen in the atmosphere had one other important outcome: It opened the door for the evolution of aerobic organisms.

Geologic evidence suggests that oxygen was present in small amounts in the atmosphere about 2.4 billion years ago. However, oxygen-releasing photosynthesis would have been present some time earlier, since the first oxygen produced would have immediately combined with elements in Earth’s crust to form oxides of various kinds. Once oxygen became a significant component of the atmosphere, the oxygen molecules also would have reacted with one another to form ozone (O3). Ozone collected in the upper atmosphere and acted as a screen to prevent most of the ultraviolet light from reaching Earth’s surface. The reduction of ultraviolet light diminished the spontaneous formation of complex organic molecules. It also reduced the number of mutations in cells. In an oxidizing atmosphere, it was no longer possible for organic molecules to accumulate over millions of years to be later incorporated into living material.

The Origin of Aerobic Respiration

The appearance of oxygen in the atmosphere also allowed for the evolution of aerobic respiration. Because the first heterotrophs were, of necessity, anaerobic organisms, they did not derive large amounts of energy from the organic materials available as food. With the evolution of aerobic heterotrophs, there could be a much more efficient conversion of food into usable energy. Aerobic organisms would have a significant advantage over anaerobic organisms. They could use the newly generated oxygen as a final hydrogen acceptor and, therefore, generate many more adenosine triphosphates (ATPs) from the food molecules they consumed.

The Establishment of Three Major Domains of Life

Biologists have traditionally divided organisms into kingdoms, based on their structure and function. However, because of their small size, it is very difficult to do this with microscopic organisms. However, advances in the ability to decode the sequence of nucleic acids made it possible to look at the genetic nature of organisms without being confused by their external structures. Biologist Carl Woese studied the sequences of nucleotides in the ribosomal RNA of many kinds of prokaryotic cells commonly known as bacteria and compared their similarities and differences. As a result of his studies and those of many others, a new concept of the relationships between various kinds of organisms has emerged. It is now clear that the bacteria that previously had been considered a group of similar organisms, are actually two very different kinds of organisms: the Bacteria and the Archaea. Furthermore, the Archaea have unique characteristics that differentiate them from other living things and share some characteristics with eukaryotic organisms.

Thus, today biologists recognize three main kinds of living things—Bacteria, Archaea, and Eucarya—that are called domains. The domains Bacteria and Archaea are both prokaryotic organisms that lack a nucleus. The domain Eucarya contains organisms that have eukaryotic cells. Within each domain are several kingdoms. In the domain Eucarya, there are four kingdoms: Animalia, Plantae, Fungi, and Protista. The process of identifying kingdoms within the Bacteria and Archaea is currently ongoing.

The oldest living things gave rise to two major types of prokaryotic organisms (Bacteria and Archaea). Strangely, the domains Archaea and Eucarya share many characteristics, suggesting that they are more closely related to each other than either is to the domain Bacteria.

It appears that each domain developed specific abilities. The Archaea have very diverse metabolic abilities. Some are chemo- autotrophic and use inorganic chemical reactions to generate the energy they need to make organic matter. Often, these reactions result in the production of methane (CH4), and these organisms are known as methanogens. Others use sulfur and produce hydrogen sulfide (H2S). Others use reactions with ammonia, hydrogen gas, or metals to provide themselves with energy. Some do a form of photosynthesis but do not release oxygen. Many of these organisms are found in extreme environments, such as hot springs, or in extremely salty or acidic environments. However, it is becoming clear that they also inhabit soil, the guts of animals, and are particularly abundant in the ocean.

The Bacteria developed many different metabolic abilities. Today, many Bacteria are heterotrophic and use organic molecules as a source of energy. Some of these heterotrophs use anaerobic respiration, whereas others use aerobic respiration. Other Bacteria are autotrophic. Some, such as the cyanobacteria, carry on photosynthesis, whereas others are chemosynthetic and get energy from inorganic chemical reactions similar to Archaea.

The Eucarya are the most familiar and appear to have exploited the metabolic abilities of other organisms by incorporating them into their own structure. Chloroplasts and mitochondria are both bacterialike structures found inside eukaryotic cells.

The Origin of Eukaryotic Cells

Biologists generally believe that eukaryotes evolved from prokaryotes. Two major characteristics distinguish eukaryotic cells from prokaryotic cells. Eukaryotic cells have their DNA in a nucleus surrounded by a membrane and have many kinds of membranous organelles. The most widely accepted theory of how eukaryotic cells originated is the endosymbiotic theory. The endosymbiotic theory states that present-day eukaryotic cells evolved from the uniting of several types of primitive prokaryotic cells. It is thought that some organelles found in eukaryotic cells may have originated as free-living prokaryotes. For example, mitochondria and chloroplasts contain DNA and ribosomes that resemble those of bacteria. They also reproduce on their own and synthesize their own enzymes. Therefore, it has been suggested that mitochondria were originally free-living prokaryotes that carried on aerobic respiration and chloroplasts were free-living photosynthetic prokaryotes. If the combination of two different cells in this symbiotic relationship were mutually beneficial, the relationship could have become permanent (figure 19.9).

FIGURE 19.9. The Endosymbiotic Theory

This theory proposes that some free-living prokaryotic bacteria entered a host cell and a symbiotic relationship developed. Mitochondria appear to have developed from certain aerobic bacteria and chloroplasts from photosynthetic cyanobacteria. Once eukaryotic cells were present, the subsequent evolution of more complex protozoa, algae, fungi, plants, and animals could take place.

Although endosymbiosis explains how many of the membranous organelles may have arisen in eukaryotic cells, the origin of the nucleus is less clear. There are currently two ideas about how the nucleus came to be. One suggests that the nucleus formed in the same way as other organelles. An invading cell with a membrane around it became the nucleus when it took over the running of the cell from the cell’s original DNA. The alternative hypothesis is that prokaryotic cells developed a nuclear membrane on their own from membranes in the cell. In other words, an increase in the number of membranes within prokaryotic cells could have produced an envelope that enclosed the DNA of the cell.

When the endosymbiotic theory was first suggested, it met with a great deal of criticism. However, continuing research has uncovered several other instances of the probable joining of two different prokaryotic cells to form one. In addition, it appears that endosymbiosis occurred among eukaryotic organisms as well. Several kinds of eukaryotic red and brown algae contain chloroplast-like structures, which appear to have originated as free-living eukaryotic cells.

The endosymbiotic theory is further supported by DNA studies. It is now clear that over their long evolutionary history, the genes within any one species of organism may appear to have arisen from several sources. We know that viruses carry genes from one organism to another, different species of bacteria can exchange genetic information, and parasitic organisms use their DNA to manipulate their hosts. The incorporation of entire cells with their DNA into other cells would also bring about the transfer of genes from one species to another and result in cells that have DNA from a variety of sources. Figure 19.10 summarizes current thinking about how endosymbiosis has been involved in the evolution of various kinds of organisms. Table 19.1 summarizes the major characteristics of these three domains.

FIGURE 19.10. Endosymbiosis and the Evolution of Eucarya

Primary endosymbiosis involves the development of a symbiotic relationship between a prokaryotic cell and another cell. Mitochondria and some chloroplasts are thought to have developed in this way. As scientists looked closely at the chloroplasts in different kinds of photosynthetic organisms, it appears that some are the result of endosymbiosis between eukaryotic organisms. For example, it appears that ancestors of Euglena gained their chloroplasts by engulfing the cells of a green alga. Once engulfed, the nucleus and mitochondria of the green alga became nonfunctional, but the chloroplast remained as the functioning chloroplast in the Euglena cell. Because the endosymbiotic relationship developed between two eukaryotic cells, it is called secondary endosymbiosis. The illustration shows two other examples of secondary endosymbiosis and one of tertiary endosymbiosis.

TABLE 19.1. Summary of Characteristics of the Three Major Domains of Life

DOMAIN

Characteristics

Bacteria

Archaea

Eucarya

Cell Structure

Few membranous structures

Few membranous structures

Many kinds of membranous organelles are present in cells.

There is no nuclear membrane.

There is no nuclear membrane.

A nuclear membrane is present.

Chloroplasts are probably derived from cyanobacteria and entered cells through endosymbiosis.

Mitochondria are probably derived from certain aerobic bacteria and entered cells through endosymbiosis.

Metabolic Activity

Some Bacteria are

chemoautotrophs that use energy from inorganic chemical reactions to produce organic molecules.

Most Archaea are

chemoautotrophs that obtain energy from inorganic reactions to make organic matter.

Most Bacteria are anaerobic heterotrophs.

There are few heterotrophs.

A few Eucarya use only anaerobic respiration—fungi, some protozoa

Many Eucarya have tissues that use anaerobic respiration—muscle

Some Bacteria are aerobic heterotrophs.

Nearly all Eucarya have mitochondria and use aerobic respiration.

Chlorophyll-based, oxygengenerating photosynthesis was an invention of the cyanobacteria.

Plants and algae have chloroplasts and use photosynthesis in addition to aerobic respiration.

Evolutionary Status

Probably related to the first living thing

Some live at high temperatures and are probably ancestral to Archaea.

Probably derived from Bacteria

Archaea probably have a common ancestor with Eucarya.

Eucarya probably have a common ancestor with Archaea.

The common evolutionary theme is the development of complex cells through endosymbiosis of other organisms.

Ecological Status

Major role as photosynthesizers in aquatic environments

Major role as photosynthesizers on terrestrial and in aquatic environments

Major category of decomposers

Archaea are typically found in extreme environments.

Dominant form of life today

Many are pathogenic.

None have been identified as pathogenic.

Various eukaryotes fill ecological roles of producer, consumer, pathogen, and decomposers.

The Development of Multicellular Organisms

Following the development of eukaryotic cells, there was a long period in which single-celled organisms (both prokaryotic and eukaryotic) were the only ones on Earth. Eventually, organisms developed that consisted of collections of cells. At first, these collections may have been very similar to modern algae in which there was very little specialization of cells (figure 19.11a). However, eventually some cells within organisms became specialized for specific tasks, and the many kinds of multicellular algae, fungi, plants, and animals developed (figure 19.11b).

FIGURE 19.11. Simple and Complex Algae

(a) Zygnema is a simple alga, which forms long, hairlike strands a few millimeters long, in which all the cells are identical. (b) The kelp, Macrocystis, is a much more complex alga, with specialized structures, such as stalks, leaf-like blades, and gas-filled bladders. It can be several meters long.

19.5. CONCEPT REVIEW

13. What organisms were probably responsible for the development of an oxygen-containing atmosphere?

14. What evidence supports the theory that eukaryotic cells arose from the development of an endosymbiotic relationship between primitive prokaryotic cells?

15. Why is it unlikely that organic molecules would accumulate in the oceans today?

16. List two significant biologically important effects caused by the increase of oxygen in Earth’s atmosphere.

17. In what sequence did the following things happen: living cell, oxidizing atmosphere, respiration, photosynthesis, reducing atmosphere, first organic molecule?

18. List two distinguishing characteristics of each of the following domains: Bacteria, Archaea, and Eucarya.