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
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common. They were unicellular (YEW-nih-sell-yew-lar) or ‘‘single’’ (uni-) ‘‘celled’’ (cellul) organisms.
Around 1.5 billion years ago, the firstmulticellular(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 unicellu- lar organism carried out all of the general processes of life. But with the coming of multicellular creatures, specializationof body structure and func- tion 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 fertilizedovum(OH-vum), or ‘‘egg’’ cell. A multicellular organ- ism results when the fertilized ovum undergoes repeated cell division and
differentiation (dif-er-en-she-AY-shun) – ‘‘a process of becoming different’’ or specialized.
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 con- sidered the most primitive members of the Plant Kingdom. They have no roots, stems, or leaves. Modern multicellular algae can form huge floating seaweeds, however.
Aplantor ‘‘sprout’’ is an organism that contains the substancechlorophyll
(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 sun- light, 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.
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 photo- synthesis. Many fungi are pathogenic (PATH-oh-jen-ik) or ‘‘disease’’ (path) ‘‘producing’’ (gen) for both plants and animals. Common examples are the
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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.
‘‘Well, what about the animals? What about the dinosaurs?’’ you may now be asking yourself. Ananimalis 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.)
The oldest animal fossils are around 600 million years old. These first animals were multicellular ocean-dwellers, such as jellyfish, corals, and sea- worms. 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 thelate Pre-Cambrian (pree-KAM-bree-un)Era.
About 500 million years ago, some animals becamevertebrates, developing a backbone. Among the first such vertebrates were jawless fishes. One hun- dred million years later, many animals joined the plants and fungi in coming out of the water to live on land.
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 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 is very vivid in people’s minds, because it is often nick- named theAge 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
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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 thedinosaurs, or ‘‘terrible’’ (dino) ‘‘lizards’’ (saurs), that comple- tely 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 dino- saurs 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).
‘‘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 mass extinction of all the dinosaurs led to theCenozoic(sen-uh-ZOH- ik) or ‘‘new’’ (ceno) ‘‘life’’ (zo) Era. The Cenozoic Era is sometimes nick- named 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. Amammalis an animal that nurses its young with its ‘‘breasts’’
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Fig. 3.4 ‘‘Mother love’’ and the duck-billed dinosaur.
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(mamma). In addition, mammals are covered with hair and areendothermic
(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, com- pared to reptiles and amphibians. They could find morehabitat(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 – swim- ming 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 impor- tance’’ (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.