Introduction to the First Living Organisms
We will probably never know for sure exactly how life began – Why? The first living things (such as primitive bacteria) left behind no fossil record . Soft-bodied micro-organisms, such as the ancient bacteria, had no hard parts. Hence, their remains were rarely preserved.
Fossil means “to dig.” Thus, the fossil record consists of the remains of ancient living things that have been preserved and “dug” up. With no hard body parts preserved, then, the first living creatures essentially left no fossils to be “dug” up later!
Nevertheless, many ancient soft-bodied bacteria created stromatolites (stroh- MAT -uh- lights ) – “layered rocks.” Even today, huge mounds of bacterial colonies live in shallow ocean water. Fine particles of dirt, calcium, and other minerals in the seawater, collect as sediment upon the colonies. This sediment eventually arranges into thin “layers” ( stromas ) streaking through each bacterial mound. As the layers or stromas become calcified (hardened with calcium sediment), they turn into layered rocks.
Figure 3.2 shows such a stromatolite. Among the oldest-known fossils are narrow, filament (“thread”)-shaped bacteria, preserved within ancient stromatolite rock from Western Australia. They are about 3.5 billion years old.
Fig. 3.2 Ancient bacteria and their rocky stromatolite.
The First Living Organisms: Tiny Green Threads Without Any “kernels”
In the living stromatolites found on present-day Earth, the bacteria are usually bluish-green in color. This green color indicates that they engage in photosynthesis ( FOH -toe- sin -theh-sis). Photosynthesis is the use of “light” ( photo ) to “place (things) together” ( synthesis ). Specifically, photosynthesis is the process whereby certain types of organisms (usually green) use the energy in sunlight to make sugar molecules for themselves. Thus, the first ancient bacteria probably looked like tiny bluish-green threads or filaments. Further, each of these ancient, threadlike, green bacteria cells lacked a nucleus.
Fig. 3.2 Ancient bacteria and their rocky stromatolite.
In Greek, the word part for “nucleus” or “kernel” is kary ( KAIR -ee). Consequently, these ancient threadlike bacteria are often called prokaryotes ( proh-KAIR -ee-oats). The reason for this name is that, being extremely ancient, the prokaryotes probably appeared “before” ( pro -) other types of more advanced cells having a “nucleus” ( kary ).
Cells With Nuclei: In Come The “kernels”
For well over a billion-and-a-half years, it appears that the prokaryotes (photosynthetic cells without nuclei) were alone on this planet. All that time, however, these tiny green cells produced abundant amounts of oxygen (O 2 ) molecules, since oxygen is one of the main by-products of photosynthesis. The prokaryotes accomplished the critical task of making Earth’s atmosphere capable of sustaining aerobic (air- OH -bik) metabolism. This is the type of metabolism that “pertains to” (- ic ) “oxygen-or-air-using” ( aer ) “life” ( ob ).
The stage was thus set for the appearance of more complex cells, many of which had aerobic metabolisms dependent upon a ready supply of oxygen. About 2.1 billion years ago, numerous eukaryotes ( yew-KAIR -ee-oats) evolved from the prokaryotes. Each eukaryote cell has a “good” (eu-) “nucleus” (kary) surrounded by its own membrane. In addition, the eukaryote cell contains numerous other organelles, each enclosed within its own individual membrane. Besides the nucleus, another very important membrane-covered organelle is the mitochondrion ( my -toe- KAHN -dree-un). The mitochondrion carries out most of the aerobic (oxygen-using) metabolism within the eukaryote cell.
Figure 3.3 provides a summary of the above information. It shows that there are two basic types of cells – prokaryotes (cells without a nucleus) and eukaryotes (cells having a nucleus and other organelles surrounded by membranes). Further, we see that the major examples of prokaryotes now in existence are the bacteria. The cells of nearly all other organisms besides bacteria, in contrast, are eukaryotes. (A detailed discussion of cell anatomy and physiology will be provided in Chapter 5.)
Fig. 3.3 The two basic types of cells: Prokaryotes versus eukaryotes.
Unicellular Organisms Evolve Into Multicellular Ones
So far, we have traced the development of life up to 2.1 billion years ago. Both the ancient prokaryotes and eukaryotes had a critical characteristic in common. They were unicellular ( YEW -nih- sell -yew-lar) or “single” ( uni -) “celled” ( cellul ) organisms.
Multicellular Organisms
Around 1.5 billion years ago, the first multicellular ( MUL -tih- sell -yew-lar) or “many-celled” organisms appeared. This event was very important, because before this time all living organisms were generalists . Each unicellular organism carried out all of the general processes of life. But with the coming of multicellular creatures, specialization of body structure and function arrived. With many cells available, some of the cells could do one body function, while other groups of cells could become specialized to carry out other body functions.
Another fact about multicellular organisms is that they usually develop from a single fertilized ovum ( OH -vum), or “egg” cell. A multicellular organism results when the fertilized ovum undergoes repeated cell division and differentiation ( dif -er-en-she- AY -shun) – “a process of becoming different” or specialized.
Multicellular Eukaryote Organisms
The multicellular eukaryote organisms, then, are the many-celled creatures whose cells contain nuclei ( NEW -klee-eye) or “kernels.” The very first such organisms appearing in the fossil record are relatively small algae ( AL -jee) or “seaweeds,” 1.5 billion years ago. These algae were greenish in color and engaged in photosynthesis. Algae are considered the most primitive members of the Plant Kingdom. They have no roots, stems, or leaves. Modern multicellular algae can form huge floating seaweeds, however.
Plants
A plant or “sprout” is an organism that contains the substance chlorophyll ( KLOH -roh- fill ) and carries out photosynthesis for creating its energy. Chlorophyll literally means “green” ( chlor ) “leaf” ( phyll ). It is the green pigment found in leaves and other parts of plants. Chlorophyll absorbs sunlight, thereby providing the energy used by photosynthesis to make sugars for plant cells.
Paleobotanists ( PAY -lee-oh- BAHT -uh-nists) are “those who specialize in” (- ist ) “ancient” ( paleo ) “plants” ( botan ). When paleobotanists studied the fossil record, they concluded that land plants (like trees, ferns, and flowers) evolved from primitive multicellular algae, about 500 million years ago.
Fungi
Related to the plants are the fungi ( FUN -jeye). A fungus ( FUN -gus) is a plant-like organism that acts as a parasite, living on dead or living organic matter. Perhaps you immediately picture a “mushroom,” after which this group is named. The mushrooms often feed on decaying leaves. Fungi can be either unicellular (as in yeast cells), or multicellular (as in molds and mushrooms). Fungi contain no chlorophyll, so they cannot utilize photosynthesis. Many fungi are pathogenic ( PATH -oh- jen -ik) or “disease” ( path ) “producing” ( gen ) for both plants and animals. Common examples are the occurrence of white spots of mildew fungus on damp leaves, and the existence of yeast infections in the female vagina.
The first fossils of fungi are recorded at about the same time as those of land plants (460–500 million years ago). One can reasonably speculate that the first fungi were probably pathogenic parasites clinging to the moist leaves of the first land plants.
Animals
“Well, what about the animals? What about the dinosaurs?” you may now be asking yourself. An animal is any “living, breathing” ( anima ) multicellular organism that is not a plant or fungus. Animals have eukaryote cells with nuclei, and they must eat other organisms or organic matter in order to survive. (We will learn much more about animals in later chapters.)
Invertebrates
The oldest animal fossils are around 600 million years old. These first animals were multicellular ocean-dwellers, such as jellyfish, corals, and seaworms. They were all invertebrates (in- VER -tuh-brits) – animals “without” ( in -) spines or “backbones” ( vertebr ). Delicate jellyfish and other invertebrate animals gracefully floated in the sea during what is technically called the late Pre-Cambrian (pree- KAM -bree-un) Era .
Vertebrates
About 500 million years ago, some animals became vertebrates , developing a backbone. Among the first such vertebrates were jawless fishes. One hundred million years later, many animals joined the plants and fungi in coming out of the water to live on land.
The Paleozoic Era and Amphibians
This general time span (from about 500–200 million years ago) is called the Paleozoic ( pay -lee-uh- ZOH -ik) or “ancient life” Era . This period saw the development of the first vertebrates (fishes, amphibians, and reptiles), land plants, insects, and vast forests of fern-like trees. Amphibians (am- FIB -ee-uns) literally “live a double life,” meaning that they can occupy “both” ( amphi -) land and water. Amphibians have a moist skin without any scales. They include frogs, toads, newts, and salamanders. Giant amphibians roamed as Kings of the Earth, long before the dinosaurs!
Reptiles
Reptiles derive their name from the Latin for “crawlers.” Most groups of reptiles (such as turtles, lizards, alligators, and crocodiles) do a lot of creeping and crawling on land. We all know that snakes are a group of reptiles that slither, however. Reptiles breathe through lungs and usually have skin that is covered by either horny plates or flat scales. The reptiles evolved from the amphibians and appeared during the middle-to-late Paleozoic Era. But they didn’t really become the dominant life form on Earth until the Mesozoic ( mess -uh- ZOH -ik) or “middle” ( meso -) “life” ( zo ) Era .
The Mesozoic Era and Dinosaurs
The Mesozoic Era is very vivid in people’s minds, because it is often nicknamed the Age of Reptiles . This period is literally “in the middle,” because it began about 200 million years ago, just after the Paleozoic Era, with its “ancient life,” and ended about 65 million years ago, just before the modern period we find ourselves in, today. The Mesozoic Era is very critical for the fossil record. The preserved bones suggest that flying reptiles likely evolved into birds, and small, shrew-like mammals arrived. Flowering plants bloomed in the forests and prairies.
But it is the dinosaurs , or “terrible” ( dino ) “lizards” ( saurs ), that completely dominated the Mesozoic Era. The dinosaurs were reptiles that lived on the land, whereas the pterosaurs ( TER -uh-sors) – “winged lizards” – were flying reptiles that took command of the sky. Mention fossils, and most people probably visualize dinosaur bones! Small wonder, since some dinosaurs were the biggest creatures ever to walk the Earth! Recent evidence suggests that particular types of dinosaurs, such as the duck-billed dinosaurs, traveled in social groups and even cared for their young, after they hatched from eggs (see Figure 3.4).
Fig. 3.4 “Mother love” and the duck-billed dinosaur.
“If the dinosaurs were so powerful, and some cared for their young, then why did they become extinct?” a curious person might ask. About 65 million years ago, Earth was nearing the end of the Mesozoic Era. According to the impact hypothesis , about this time a huge comet or asteroid crashed into the Earth at great speed. The force of impact created a massive cloud of dust and debris. This great cloud blocked most of the sunlight, killing plants and dramatically cooling Earth’s tropical climate. Plant-eating dinosaurs had nothing to eat, and died out. Thus, meat-eating dinosaurs, which could not devour plant-eating dinosaurs, became extinct as well.
The Cenozoic Era - The Age of Mammals
The mass extinction of all the dinosaurs led to the Cenozoic ( sen -uh- ZOH -ik) or “new” ( ceno ) “life” ( zo ) Era . The Cenozoic Era is sometimes nicknamed The Age of Mammals . The reason is because during this past 65 million years or so, leading up to the present day, mammals have become the dominant animals on the Earth. Mammals, like birds, probably evolved from reptiles. A mammal is an animal that nurses its young with its “breasts” ( mamma ). In addition, mammals are covered with hair and are endothermic ( en -doh- THER -mik). Endothermic organisms have “inner” ( endo -) control of their body “heat” ( therm ) or temperature. Therefore, early mammals had a much greater ability to adapt to the colder climate of the modern era, compared to reptiles and amphibians. They could find more habitat ( HAB -uh-tat) on the land and water, a wider variety of colder places to “live in” ( habit ). [ Study suggestion: Would you expect to see an alligator – a reptile – swimming in the freezing water of the Arctic Ocean? Or, would you be more likely to see a killer whale – a marine mammal – swimming there?]
Most of the main groups of mammals were in existence about 50–60 million years ago. One of these groups was the primates ( PRY -mates). Since human beings (Homo sapiens) belong to the group of primates, it is only natural that we should label ourselves as being “of first rank or importance” ( primat ). Primates include monkeys, apes, and various other creatures, as well as humans. The apelike ancestors that may have evolved into humans appeared approximately 5 million years ago. Our species, Homo sapiens, finally showed up just 100,000 to 200,000 years ago. This time has often been nicknamed The Ice Age . This is due to the fact that fossils of early humans were found amid evidence of huge glaciers in Europe.
Fossil Record Summary
Table 3.1 provides a brief summary of the major Geological ( jee -uh- LAHJ -uh-kul) or “pertaining to Earth-study” Eras . Observe from the table that there are four Eras: the Pre-Cambrian, Paleozoic, Mesozoic, and Cenozoic Eras. Not shown in the table are a number of Periods , into which each of the eras is subdivided. Finally, the most recent (Cenozoic) Era has two periods, each of which is subdivided into a number of Epochs ( EP -uks). Since this book just provides an overview, the specific periods and epochs have not been identified. [ Study suggestion: Picture three steps of geological time going down, big to smaller to smallest. The top step is the Era. The middle step is the Period. And the third step is the Epoch.]
The relative amount of time covered in each of the four major eras is easily visualized within the FOSSIL RECORD CLOCK (Figure 3.5). Recall that each mark on this CLOCK represents a time span of 0.25 bya (billion years ago), or 250 mya (million years ago). Therefore, the appearance of human beings a mere 100,000 years ago is so recent in Earth’s overall history, that it occurred just the last second or so before our model CLOCK strikes its end at midnight!
Table 3.1 The four major eras and their characteristics.
Fig. 3.5 The fossil record clock.
Practice problems for these concepts can be found at: From Dawn To Darwin Evolution Test
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