Two Types of Cell
The domain concept of biological organization is relatively new. As recently as the mid-twentieth century, two kingdoms—plant and animal—were widely accepted as describing the most significant split in the biological world. Every living thing was classified as either a plant or an animal. Subsequently, three additional kingdoms were recognized.
Only in the late twentieth century did it become clear, based on molecular and other evidence, that distinctions at the level of the kingdom did not acknowledge the most fundamental differences among organisms. A higher category, the domain, was therefore posited.
The general acceptance of the domain concept by the scientific community was an acknowledgment that, at least according to current knowledge, the differences between the prokaryotic organisms and the eukaryotic ones, and further, the split within the prokaryotic organisms, are the major dividing lines in the biological world.
Bacteria is the domain of prokaryotic organisms that are considered to be true bacteria, and Archaea is the domain of prokaryotic organisms able to live in extreme environments. Eukarya differs from the prokaryotic domains in basic characteristics of cellular organization, biochemistry, and molecular biology.
Further, unlike the prokaryotic organisms, many of the Eukarya are truly multicellular. Eukaryotic cells,which are structurally more complex than prokaryotic ones, have many of their cellular functions segregated into semiautonomous, membrane-bound cell regions, called organelles.
The principal organelle is the nucleus, which contains the geneticmaterial, deoxyribonucleic acid (DNA). In prokaryotic organisms, in contrast, the DNA is not segregated from the rest of the cell.
Other distinguishing organelles in eukaryotic cells include the mitochondria. These are the sites of respiration, in which energy is generated by breaking down food, in the presence of oxygen, into water and carbon dioxide. The plants and the algae have additional organelles, the plastids.
The most common plastid is the chloroplast, which contains chlorophyll, the key molecule that allows algae and plants to manufacture their own food from carbon dioxide and water, by photosynthesis. In contrast, in those bacteria that are photosynthetic, chlorophyll is not confined within an organelle.
Prokaryotic cells are much older than eukaryotic cells and had a long reign in the primordial seas before one of them, probably a member of the domain Archaea, gave rise to the first eukaryotic cell, between 2.5 billion and 1 billion years ago. This was at least a billion years after life had arisen. The first eukaryotic cell lacked mitochondria and chloroplasts.
Subsequently, two kinds of prokaryotic organisms belonging to the domain Bacteria took up residence, as symbionts, inside early eukaryotic cells and eventually became so dependent on their hosts that they could no longer live on their own. These socalled endosymbionts developed into mitochondria and plastids.
Mitochondria, which were acquired before chloroplasts, arose from small bacteria that were heterotrophic: They obtained their food from other organisms rather than manufacturing it themselves. Chloroplasts arose from bacteria known as cyanobacteria, which were autotrophic, manufacturing their food themselves by photosynthesis.
Thus, the evolution of the eukaryotic cell involved three prokaryotic cells—the original archaean host cell and two kinds of endosymbiotic bacteria. The early eukaryotic organisms were single-celled and are classified in a group called protists.
From their beginning as single-celled protists, eukaryotic organisms evolved rapidly. The first multicellular eukaryotic organisms appeared about 800 million years ago, during the Precambrian era, and developed into three great lineages: the fungi, plants, and animals. Scientists have accorded each of these groups “kingdom” rank within the domain Eukarya, as the kingdoms Plantae, Animalia, and Fungi.
In addition, the protists, which have living representatives today, are considered to constitute a fourth kingdom, the Protista, within the Eukarya. The Protista consist of the predominantly unicellular phyla and some of the multicellular lines associated with them. All four of the kingdoms within the Eukarya arose in the sea. Transition to the land occurred later.
The three multicellular kingdoms—plants, animals, and fungi—each probably descended from a separate ancestor from among the protists, and thus each of these lineages constitutes a relatively well-defined natural kingdom. The protists, however, are something of a “catch-all” kingdom.
They consist of a variety of lineages, which include both photosynthetic organisms, the algae, and nonphotosynthetic ones. Because the algae are capable of photosynthesis, older classification schemes lumped them with the plants. Scientists think that many kingdoms will ultimately be recognized among the Protista.
The protist that gave rise to the plant lineage was probably a now-extinct member of the family Charophyceae, a group of specialized, aquatic, multicellular green algae (phylum Chlorophyta) that includes members living today. Like the algal protists, plants are autotrophic, but plants have more complex, structurally integrated bodies than do the algae.
In contrast to the algal protists and the plants, the nonphotosynthetic protists, as well as all the fungi and the animals, obtain their food heterotrophically, from other organisms. The fungi, although lacking chloroplasts and photosynthetic pigments, were once, like the algae, classified within the plant kingdom. This was partly because, like plants, fungi are sedentary.
The fungi, however, have little in common, nutritionally or structurally, with plants and are now recognized as an independent evolutionary line within the Eukarya. Molecular evidence indicates that the fungi are actually more closely related to animals than to plants.
Colonization of the Land
Although all of the lineages that are now recognized as kingdoms within the Eukarya originated from aquatic organisms, the Eukarya eventually achieved great success on the land. Multicellularity helped these organisms make the transition to an environment of earth and air, which was more complex and demanding than the relatively uniform conditions of the sea.
With their many cells, the Eukarya were able to develop specialized structures for coping with this new environment. The evolution of plants shows a trend toward structures specialized for anchorage, photosynthesis, and support.
This trend eventually led to the development of complex plant bodies, with roots, leaves, and stems, allowing the plants as a kingdom to be fully terrestrial, not aquatic. Had it not been for plants’ pioneering of the land, animals could not have become established there, because plants form the base of terrestrial animals’ food chain.
Plants may have first invaded the land sometime in the Ordovician period of the Paleozoic era, 510 million to 439 million years ago. Forms resembling modern land plants arose in the Late Silurian period of the Paleozoic, more than 408 million years ago.
By the close of the Paleozoic’s Devonian period, about 360 million years ago, plants had diversified into a wide variety of shapes and sizes, from small creeping forms to tall forest trees.
In fossils of early plants, fungi are often found in close association with the roots. Some scientists think that plants were able to colonize the land only because they developed symbiotic relationships with such representatives of the fungal kingdom.
Scientists also think that fungi, in turn, may have been able to make the transition to land only because of their close relationship with plants. According to this view, the fungi helped the plants gain a terrestrial foot hold by absorbing water and mineral nutrients from the poorly developed soils of that time and passing the mon to their plant partners.
The plants provided the fungi with sugars that the plants had manufactured photosynthetically. This is much the way that relationships between plant roots and certain fungi work today.
These symbiotic, so-called mycorrhizal relationships are characteristic of the vast majority of plants that dominate the modern world—the plants having vascular, or conducting, tissues. Of these plants, the most important by far are the angiosperms, or flowering plants (phylum Anthophyta).