Supplemental Lecture (97/05/10 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Ascent of Mammals
    1. A list of vocabulary words is found toward the end of this document
    2. We, Homo sapians, are mammals. Consequently, as a story, the origin of man may properly begin with a story of from where the mammals came. By necessity, such a story touches on the rise of multi-cellular animals, the vagaries of fossilization, the wonder of the reptilian egg, the impact of large bodies of extraterrestrial origin with an earth teaming with life, etc. This lecture ends with the beginning of the age of mammals some 65 million years ago. In the next lecture, the descent of man, we consider a specific mammalian lineage from which H. sapian directly arose.
  2. Timeline
    1. Below is a trace through time of the origin of mammals including various key events. Time is in units of millions of years before present.

    millions of years
    before present

    event

    4500

    origin of earth

    3500

    oldest fossil bacteria

    3000+

    appearance of photosynthesis

    2000+

    oxygen containing atmosphere

    1500

    appearance of eucaryotes

    800+

    appearance of metazoans

    600+

    appearance of chordates

    600

    appearance of organisms with hard parts

    600

    end of precambrian eon

    600

    85% of earth's history already over

    600

    start of phanerozoic eon

    600

    beginning of Cambrian period

    550

    appearance of jawless fish

    500

    end of Cambrian period

    450

    appearance of terrestrial plants

    420

    appearance of jawed fish

    350

    appearance of amphibians on land

    300+

    appearance of reptiles

    300

    appearance of insects

    250

    appearance of mammal-like reptiles

    200+

    appearance of mammals

    200+

    appearance of dinosaurs

    200

    start of Jurassic period

    150

    end of Jurassic period

    150

    appearance of birds

    80

    appearance of tree shrew-like protoprimates

    65

    meteor strikes earth leading to extinction of dinosaurs

    65

    end of the Cretaceous period

    65

    beginning of the Tertiary period

    65

    beginning of the "age of mammals"

  3. Origin of life
    1. Simplicity through complexity:
      1. Life is defined based in part upon assesments of complexity.
      2. Very often even systems capable of responding to their environment, replicating, and evolving are not sufficiently complex to be universally defined as living system (e.g., the viruses).
      3. We assume that simple systems have a higher probability of spontaneous genesis than do more complex systems.
      4. Therefore, we define life as only having arisen once some threshold of complexity has been reached, and we assume that simpler life-like systems existed prior to the existence of systems that would universally be recognized as alive.
      5. However, once a minimum level of complexity has been achieved, further evolution and adaptation must either:
        1. result in no change in complexity
        2. a decrease in complexity which renders the system no longer alive (by some definition of alive)
        3. or must demonstrate an increase in complexity
      6. Life, thus, retains some degree of propensity to evolve from less complex to more complex forms.
      7. Furthermore, this order of evolutionary "progression" inevitably implies that, all things being equal, a given "more complex" niche is more likely to be open to exploitation than a given "less complex" niche, regardless if "more complex" or "less complex" niches are more or less prevalent in absolute terms.
    2. Evolution not just increases in complexity:
      1. With time life appears to increase in complexity, with some extant organisms displaying dramatically more complexity than probably existed three, two, one billion or perhaps even 100 million years ago.
      2. However, the fact that increased complexity inevitably occurs does not mean that evolution inevitably increases the complexity of any one lineage.
      3. Instead, lineages follow paths which are unique to their particular circumstances.
      4. For reasons similar to the likelihoods of advantageous mutations occurring being much lower than the likelihoods of disadvantageous mutations occuring, it is far easier for a lineage to show either stability or a decline in complexity over time, than to show increased complexity. This is true unless there exist open niches which an obtainable increase in complexity by a given a lineage will result in their successful exploitation.
    3. New niches:
      1. The ascent of mammals, in very broad terms, in fact can be followed through a number of key evolutionary innovations, each of which appears to have resulted either in the exploitation of a previously open (or newly opened) niche, or in the continued exploitation of a subtlely changing niche.
      2. Thus, in tracing of the evolution of animals we will pay attention both to how complexity appears to be increasing and to how these changes likely resulted in an increased potential for environmental exploitation.
    4. Selection for lowered complexity:
      1. A second reason for why increases in complexity is not the whole story has to do with the non-linear nature of ecological interactions.</P.
      2. In short, the exploitation of niches via the harnessing of greater complexity tends to open up even more niches requiring the harnessing of much less complexity.
      3. Thus, the paradox of biology is that while biologists have been so fixated on the seemingly inevitable increase in the complexity of individual organisms with time, in fact it is the less complex organisms which have been prospering the most.
      4. Witness, for example, how one species of tree can support tens, hundreds, or thousands of species of animals, while each of those animals can support tens, hundreds, or thousands of less complex symbionts.
    5. Age of bacteria:
      1. Thus, despite the fact that we, as animals, have a strong tendency of assessing the progression of evolution in terms of the increasing complexity of animals, in fact what has really been going on is that the bacteria were the dominant life forms on the planet when life first arose, have been the dominant life forms since then, and will likely continue to remain the dominant life form on this (or any) planet until life no long exists.<DD
      2. "Life on earth evolved quickly and is as old as it could be. . . For reasons related to the chemistry of life's origins and the physics of self-organization, the first living things arose at the lower limit of life's conceivable preservable complexity. Call this lower limit the 'left wall' for an architecture of complexity. Since so little space exists between the left wall and life's initial bacterial mode in the fossil record, only one direction for future increment exists---toward greater complexity at the right. Thus, every once in a while, a more complex creature evolves and extends the range of life's diversity in the only available direction. In technical terms, the distribution of complexity becomes more strongly right skewed through these occasional additions. But the additions are rare and episodic. They do not even constitute an evolutionary series but form a motley sequence of distantly related taxa, usually depicted as eukaryotic cell, jellyfish, trilobite, nautiloid, euryperid (a large relative of horseshoe crabs), fish, an amphibian such as Eryops, a dinosaur, a mammal and a human being. This sequence cannot be construed as the major thrust or trend of life's history. Think rather of an occasional creature tumbling into the empty right region of complexity space. Throughout this entire time, the bacterial mode has grown in height and remained constant in position. Bacteria represent the great success story of life's pathway. They occupy a wider domain of environments and span a broader range of biochemistries than any other group. They are adaptable, indestructible and astoundingly diverse. We cannot even imagine how anthropogenic intervention might threaten their extinction, although we worry about our impact on nearly every other form of life. The number of Escherichia coli cells in the gut of each human exceeds the number of humans that has ever lived on this planet. . . This is the 'age of bacteria'---as it was in the beginning, is now and ever shall be." (Gould, 1994)
  4. Evolution of animals
    1. Progression:
      1. Animal evolution, the clearly bush-like in its phylogenetic reconstruction, nevertheless can be described somewhat in terms of a linear progress of probably not inevitable events beginning with unicellularity and ending with a not terribly modest nod to ourselves in a highlighting of the ultimate prominence of mammals.
      2. The majors steps in this "advance" are chronicled in order as follows:
        1. unicellularlity
        2. multicellularity
        3. mobility (animals)
        4. chrodates
        5. hard parts
        6. vertebrates
        7. fish
        8. jaws
        9. bones
        10. lobe-finned fish
        11. lungs
        12. amphibians
        13. amniote egg
        14. reptiles
        15. mammal-like reptiles
        16. dinosaurs
        17. mammals
        18. prominance of mammals
  5. Metazoans
    1. Multicellular/animals:
      1. Metazoans are organisms, particularly animals (other than sponges) consisting of more than one cell.
      2. This is in contrast to, for example, the unicellular protozoa.
    2. Long evolutionary lag:
      1. Multicellularity is thought to have had its origins long after (billions of years) the origin of life.
      2. This lag suggests either that:
        1. the sophistication of cells took a long while to reach the point where multicellularity was possible
        2. the environment did not offer an advantage to a multicellular way of life prior to the time coinciding with their appearance
    3. Origins of multicellularity:
      1. The first multicellular creatures were probably blue-green algea (cyanobacteria) and one can imagine cells simply not separating after division as great photosynthesizing mats bask in the sun.
      2. Perhaps there was differentiation among these cells such that overall the mat was more efficient at collecting light than might a similar number of individual cells.
      3. Greater overall efficiency probably doesn't equate with an overall increase in efficiency of each individual cell. However, if all of the cells are genetically identical, one for all and all for the common genotype should lead to increased reprodutive success for a cell's genotype even if not for that particular cell itself.
      4. Thus, if a mat is more efficient at collecting resources (light) than individual cells, per cell there might be more resources to put into the production and protection of dispersing progeny, thus perhaps explaining the evolutionary advantage acrued by these algea mats.
    4. Origins of metazoa:
      1. The occurrence of multicellularity in relatively imobile species is easy enough to imagine. A species that need not move need not accomplish too great a level of coordination among cells in order to achieve an advantage with multicellularity. Indeed, dispersal can still be accomplished employing a mobile unicellular stage.
      2. What, then, drove the evolution of mobile metazoans (animals)?
      3. Perhaps multicellularity allowed greater or more controlled mobility, or simply feading efficiency, independent of the ability of the organism to move.
      4. Perhaps the long gap between the occurrence of multicellularity in animals is due in part to the presence and then removal of a constraint such as that which may have occurred with the evolution of aerobic respiration. Thus, armed with a more efficient biochemistry, animals could have been more free to take advantage of inovations that allowed mobility.
      5. Perhaps algae mats were followed by cellular blobs capable of eating algae mats.
    5. Highly differentiated consumers of producers:
      1. Some level of cellular differentiation within algae eating blobs may have made eating mats more efficient.
      2. Finding mats may not at first have been a problem (since mats might have been very abundant in a world lacking predators). As mats became less abundant, increasingly mobile predator variants may have come to the fore (years later mobility also had important paleontological ramifications since these very early animals appear to be known only because they left numerous and distinctive trails).
      3. Selection would probably favor those metazoans capable of sensing their environment with increased sophistication (seeing, for example).
    6. Consumers of consumers:
      1. An increasingly numerous fauna could lead to the selection for individuals capable of consuming fauna.
      2. This, perhaps, led to a development of countermeasures among the faunal victims of predation (i.e., those best able to defend themselves would have a higher likelihood of survival and passing on progeny)
      3. Predating modile consumers is likely more difficult than predating sessile producers thus selecting for increased complexity among predators.
      4. Coevolution of predators and prey (including producers) likely strongly influenced the evolution of ever greater complexity among both producers and consumers.
  6. Precambrian eon
    1. Faunal fossil record, part I:
      1. The Precambrian eon eon represents that portion of earth's history during which life arose, though prior to which signficant fossilization occurred owing to a relative dearth of organisms possessing hard parts.
    2. Evolution of hard parts:
      1. The Precambrian eon came to a close, by definition, as some fraction of fauna developed such things as (presumably protective) external skeletons and shells.
      2. This led to dramatic increases is the potential for fossilization and early geologists used this break from the past (no fossils to suddenly many fossils) as a world-wide relative date.
    3. "Animals began to crawl about on the seafloor at a time when the only predators were small and soft. They (animals) did so by becoming large and flat so that muscular energy could be transmitted more effectively to the ground. However, this shape became impractical once animals with teeth evolved, because a tooth-covered proboscis could rip an unprotected soft body to ribbons. The solution was to cover up with an armor made either of overlapping mineral scerites or another kind of hard exterior, or alternatively, to hide from predators by burrowing into sand or mud. At this time, it was possible to avoid being swallowed whole by having spiny projections that would stick in the sides of a predator's gut; these are the weapons of defence of early trilobites and other arthropods . . . However, when predators became more efficient, the possession of larger, continuous shells rather than overlapping sclerites became a more effective means of survival." (p. 86, Runnegar, 1992)
  7. Cambrian period [Cambrian explosion]
    1. Faunal fossil record, part II:
      1. The Cambrian period represents a time of explosive, fossilized faunal diversification.
      2. The Cambrian period represents the most dramatic period of paleontologically tractable diversification in the history of life.
      3. As Gould (1994) puts it, "subsequent history of animal life amounts to little more than variations on anatomical themes established during the Cambrian explosion."
    2. Monophyly or polyphyly?
      1. We cannot be certain that this diversification (often described as a time of invention of novel animal "body plans") actually occurred during the Cambrian period.
      2. Alternatively, it may simply have been a time during which diverse animal lineages, probably by necessity, more or less simultaneously developed easily fossilized armenants.
      3. Note how this debate can be summarized as one between champions of a monophyletic and champions of a polyphyletic Cambrian origin of highly fossilizable species.
      4. Thus, if these newly fossilized species are monophyletic, then it is likely that armor arose in a single lineage which such that an armored species is the common ancestor to all subsequent hard part containing species (i.e., hard parts would be considered to be homologous).
      5. Alternatively, it could be that the common ancestor to all armored species was instead an organisms which did not possess armor. This explanation posits that diversity of animal structures evolved prior to the evolution of hard parts, thus implying that hard parts are a product of convergent evolution.
    3. Burgess shale:
      1. How to decide? We need ever more fossils, especially of soft bodies precambrian fauna.
      2. There exist, for example, fossils fo many Cambrian animals not possessing easily fossilized structures (i.e., having soft bodies) in a deposite known as the Burgess Shale deposites found in the Canadian Rockies.
      3. These organisms appear to have been buried by very fine sediment very quickly thus resulting in the very slow decay of soft parts, the impression of these soft parts in the sediment prior to their decay, and the retention of many fine structures due to the fineness of the sediment.
  8. Animals [fauna]
    1. Characteristics:
      1. All animals are:
        1. multicellular
        2. employ the structural protein collagen
        3. reproduce by a method that employs meiosis1
        4. have a nervous system composed of neurons2
      2. 1circumstances indicate that absence of meitotic reproduction in various animals is likely a derived trait.
      3. 2sponges are the exception.
  9. Chordates [notochord]
    1. Spinal chords:
      1. Chordates areanimals having a ventral nerve chord (spinal chord) but which do not necessarily have that chord sheathed in a segmented backbone (the defining characteristic of the vertebrates).
      2. That is, while all vertebrates are chordates, not all chordates are vertebrates. (e.g., tunicates (sea squirt) and lancelets).
    2. Those chordates which lack a backbone do possess a flexible, internal rod (notochord) along their spinal chord equivalent.
  10. Vertebrates
    1. Vertebrates are "cephalized, sensate, bilaterally symmetrical, motile, coelomate gnathostome having a segmented endoskeleton, a dorsal hollow nerve chord, and a ventral gut." (p. 119, Ostram) Got that?
    2. Having a specialized body area in which neural and sensory organs are concentrated (i.e., a head).
    3. Having structures through which knowledge about the external world is transduced into modified internal states (e.g., the sense of smell, taste, sight, etc.).
    4. Possession of a single media axian such that one and only a single plane is capable of dividing the body into two highly similar halves (i.e., the left side is an approximate mirror image of the right side).
    5. The ability to move.
    6. Coelomate:
      1. A body cavity within which our gastrointestinal system lies.
      2. "Tube within a tube."
      3. This is to be contrasted with a state in which the internal "surface" of gastrointestinal system (i.e., that side facing the body) is continuous with "surface" of the body facing it.
      4. Instead, in vertebrates these surfaces are not directly attached.
    7. Gnathostome:
      1. Hinged mouth, i.e., possessing a jaw.
      2. Note, however, that while most vertebrates are indeed gnathostomes, there exist a handful of primative jawless fish which, while certainly vertebrates, nevertheless do not possess a hinged mouth.
    8. The existance of joints (such as your elbow).
    9. Endoskeleton:
      1. An internal skeleton.
      2. To be contrasted with a shell or the external skeleton of an arthropod.
    10. Dorsal:
      1. The side where the backbone is.
      2. This is the top in most vertebrates.
      3. This is the back in humans.
      4. Think dorsal fin, the triangular fin on the backs of sharks.
    11. Our backbone (spinal column) spinal cord sheath.
    12. Ventral:
      1. The opposite side of the body from where the backbone is.
      2. The bottom in most vertebrates.
      3. This is the front in humans.
      4. Snakes, for example, slither upon their ventral surface.
    13. A digestive organ possessing both a mouth and an anus.
  11. Jawless fish
    1. Primitive vertebrates:
      1. The most primitive and likely earliest vertebrates.
      2. The jawless fish lack a bony skeleton and, obviously, jaws.
      3. These fish also lack the two pairs of fins (or legs) which are characteristic of all other vertebrates (though they may possess one pair of fins).
    2. Examples of jawless fish include the hagfish and the lampreys.
  12. Jawed fish
    1. Cartilaginous origin:
      1. A lack of a bony skeleton does not cause a lack of jaws.
      2. To put it another way, sharks lack bony skeletons.
      3. The earliest jawed fish, many of those existing to today, (and the chordate notochord) are cartilaginous rather than bony.
    2. There is some evidence for a polyphyletic origin of jaws.
  13. Bony fish
    1. A major advance shared both many modern fish as well as their terrestrial descendants is the possession of a bony skeleton.
    2. Uncertain advantage:
      1. Though apparently quite successful, the reason why a fish with a bony skeleton should (or even by necessity do) possess a selective advantage over fish with cartilagenous skeletons is not completely understood.
      2. One possibility is that bones initially were ossified cartilage which served a calcium storage purpose.
    3. Regardless of their utility in fish, it is seems certain that the evolution of a bony skeleton was a necessary preadaption to a terrestrial existence since bone, but to much less of an extent cartilage, has the necessary inflexibiliy to support vertebrates when not present in a bouyant medium.
  14. Lobe-finned fish [coelacanths]
    1. Leg preadaptation:
      1. A minor branch of the jawed fish possessing bony skeletons was a group of fish which additionally developed sturdy, skeletally supported, lobe-like fins.
      2. These are the fish considered to be the probable ancestors of all terrestrial vertebrates.
  15. Amphibians
    1. Terrestrial fish:
      1. The amphibians evolved immediately from fish.
      2. They most distinctly were (are) not limited to water in their locomotion.
      3. Roughtly, at the point where lobe-finned fish were, in a sense, effectively equally adapted to both land and water locomotion, is the point at which they are no longer considered to be fish but instead amphibians.
      4. Quite a bit of additional anatomical differences exist between fish and amphibia.
      5. Additionally, it should be noted that modern amphibians have diverged notably from their amphibian ancestors.
    2. Water copulators:
      1. The amphibians make up a paraphyletic taxon so long as reptiles and the various descendants of the reptiles are excluded.
      2. One key difference between amphibians and reptiles is the latter's retention of typically strong ties to water.
      3. To be truly free from the aquatic life, an animal must not only develop various adaptations which prevent the rapid desication of their body, but also a strategy for the development of embryos away from the aquatic environments.
      4. Even today, the majority of amphibians continue to live either within water or at least in very moist environments, and the majority of the rest must return to water to lay their eggs.
      5. Particularly, most amphibia minimally must return to the water to mate in a manner reminiscent of that of fish, i.e., via the external fertilization of layed eggs.
      6. Thus, amphibians not surprisingly, tend to inhabit environments which constitute air-water (especially fresh water) interfaces.
  16. Reptiles
    1. Freedom from water:
      1. Though amphibians (perhaps by definition) are adept at terrestrial locomotion, they nevertheless, generally, are not well adapted to the highly desiccating environment that air often represents.
      2. Instead, it is the reptiles which first possessed an ability to interact with water solely as a quench for their thirst.
      3. Among the adaptations necessary to achieve this kind of freedom include:
        1. internal fertilization
        2. mechanisms for development of embryos out of water
        3. possession of desiccation resistant skin
      4. Additional helpful adaptations include the ability to concentration wastes (urine and feces) prior to elimination, thus allowing further retention of water.
      5. Presumably the first reptiles invaded niches which nevertheless were relatively close to water, moisture, and humidity, thus allowing these various adaptations evolve relatively slowly, with increased freedom from the water achieved only in steps.
    2. The reptile clade:
      1. Among the many groups of reptiles and their descendants are the:
        1. lizards & snakes
        2. crocodiles
        3. turtles & tortoises
        4. tuatoras
        5. dinosaurs
        6. birds
        7. mammals
      2. "In the fossil record there are intermediates between (reptiles) and both amphibian and mammals, and no sharp dividing line is possible." (p. 204, Abercombie et al., 1951)
  17. Amniote egg
    1. Hard-shelled egg:
      1. The solution to the problem of needing to return to water to lay eggs was the evolution of the amniote egg.
      2. That is, a "hard-shelled, self-contained system capable of both protecting, and nourishing, the developing embryo." (p. 123, Ostrom, 1992)
    2. Peak of reptilian innovation:
      1. The lineage possessing the amniote egg gave rise to the true vertebrate conquerers of land, the reptiles.
      2. "From the hindsight of the present day, development of (the amniote egg and the associated freedom from returning to a watery environment in order to reproduce) must be considered the most important event in vertebrate history after adoption of the overall vertebrate" body plan. (p. 124, Ostron, 1992)
    3. Internal fertilization:
      1. Hand in hand with the evolution of the amniote egg was the origin of mechanisms of internal fertilization.
  18. Dinosaurs
    1. The dinosaurs were diverse reptile-like creatures that became the dominant terrestrial vertebrates reining from approximately 215 million years before present until 65 million years ago.
  19. Mammal-like reptiles
    1. Early mammals:
      1. Prior to the rise of dinosaurs, the progenitors of the mammals began diverging from the reptiles.
      2. These reptiles had mammal-like characteristics (at first subtle) that distinguished them from other contemporaneous reptiles.
      3. At their most advanced, the mammal-like reptiles possessed such mammalian traits as well-differentiated teeth, an upright posture, as well as numerous other mammal-like skeletal modifications.
  20. Mammals
    1. Dinosaur contemporaries:
      1. Skeletally true mammals (perhaps also possessing the difficult to fossilize hair and breasts that mark the mammalian lineage) presumably developed from the advanced mammal-like reptiles.
      2. Some time in the course of the development of the mammals also saw the development of endothermy (warm-bloodedness) in this lineage. as well as the various other not easily fossilized mammalian traits.
      3. These true mammals first appear in the fossil record at about the same time as the first dinosaurs.
      4. Of the two groups, the dinosaurs were the more successful, especially as large animals.
    2. Conquerors of the small terrestrial vertebrate niche:
      1. The mammals apparently were forced into roles as small animals (i.e., the big ones went extinct, natural selection favored smaller varieties) since their evolution prior to (and following) the presence of dinosaurs certainly does not suggest an inability to assume roles as large, dominant, terrestrial fauna.
      2. For the 150 million years while the dinosaurs dominated the landscape, the mammals were relegated to body sizes rarely as large as that of a house cat. There mammals would remain until the extinction of the majority of dinosaurs.
      3. Note that another way of looking at this is the mammals forced the dinosaurs out of the small animal niches.
  21. Cretaceous-Tertiary [KT] boundary
    1. Extraterrestrial dinosaur killer:
      1. The rein of the dinosaurs ended with the collision of a large extraterrestrial body with the earth.
      2. The impact of this object combined with associated temporary climatic changes apparently led to the extinction of the majority of the dinosaurs.
      3. Both events define the end of the Cretaceous period and the beginning of the Tertiary period by relative dating.
    2. Only the dinosaur lineage we call birds retained some aspect of the terrestrial dominance exhibited by the dinosaurs prior to the Cretaceous-Tertiary boundary.
    3. Survival of mammals:
      1. Why did the mammals survive while the dominant dinosaurs die?
      2. One possibility is that the mammals had adapted to their marginalized existence by becoming generalists, having large populations, having small sizes, etc. These plausibly are just the traits needed to survive a globally catastrophic extraterrestrial impact.
      3. Thus, mammals may have survived precisely because they were not the dominant terrestrial organisms of that time or, more precisely, dominates the small, generalist niche.
    4. Age of mammals:
      1. Upon the extinction of the majority of the dinosaurs, mammals became the dominant terrestrial vertebrates (the age of the reptiles had ended, the age of the mammals had begun) as they underwent an extensive adaptive radiation.
      2. This adaptive radiation continued in all its glory until anatomically modern Homo sapiens effectively destroyed the mammal nourishing environment in which she evolved (e.g., read E.O. Wilson, 1992).
  22. Vocabulary
    1. Amniote egg
    2. Amphibians
    3. Animals
    4. Bony fish
    5. Cambrian explosion
    6. Cambrian period
    7. Coelacanths
    8. Chordates
    9. Cretaceous-Tertiary (KT) boundary
    10. Dinosaurs
    11. Extinction of the dinosaurs
    12. Jawless fish
    13. Lobe-finned fish
    14. Mammal-like reptiles
    15. Mammals
    16. Metazoans
    17. Origin of life
    18. Precambrian eon
    19. Reptiles
    20. Vertebrates
  23. Practice questions
    1. A vertebrate may be defined as a motile organism which has a head (cephalized) sporting various sensory organs (sensate), has a left side which resembles its right side (bilaterally symmetrical), moves about (motile) has a bony support that encloses a dorsal nerve cord (dorsal hollow nerve chord), has a ventral gut, consists of a tube within a tube (coelomate) one end of which (the mouth) is toothed in some manner (gnathosome), and whose flesh surrounds an endoskeleton (endoskeleton). What's missing? (no, the answer is not a jaw, though that's close) [PEEK]
    2. Which came first? (i.e., are found earliest in the fossil record) (circle best answer) [PEEK]
      1. dinosaurs
      2. mammal-like reptiles
      3. tree-shrews
      4. Homo erectus
      5. the Age of the Mammals
    3. When did the Tertiary period begin (I'm looking for a number with two significant figures)? [PEEK]
    4. What is another name for a multicellular creature, especially an animal? (single word answer) [PEEK]
    5. If muscular energy is best transferred to the sea floor by broad, flat organisms, why are so few of the organisms found in the world today broad and flat?[PEEK]
    6. Despite highly relevant adaptations to a terrestrial existence such as scales and a more upright stance, the most significant adaptation accomplished by the reptiles allowing the vertebrate conquest of land was the evolution of what?[PEEK]
    7. Describe the limbs of the fish from which amphibians descended.[PEEK]
    8. Between 300 and 450 million years ago a number of key events occurred in the history of life. Name one of these events. [PEEK]
    9. What is a metazoan? [PEEK]
  24. Practice question answers
    1. joints
    2. ii, mammal-like reptiles
    3. 65 million
    4. metazoan
    5. too susceptible to predation (especially toothed predation)
    6. the amniote egg
    7. these fish had lobe-like (fleshy) fins
    8. appearance of jawed fish, appearance of amphibians on land, appearance of reptiles
    9. a multicellular life form, usually considered to be an animal. Contrast with protozoan.
  25. References
    1. Abercrombie, M., Hickman, C.J., Johnson, M.L. (1951). A Dictionary of Biology Aldine Publishing Company, Chicago.
    2. Gould, S.J. (1994). The evolution of life on earth. Scientific American October:85-91.
    3. Ostrom, J.H. (1992). A history of vertebrate success. Major Events in the History of Life. J. W. Schopf (ed.). Jones and Bartlett Publishers, Boston. pp. 119-139.
    4. Postlethwaite, J.H., Hopson, J.L. (1995). The Nature of Life. Third Edition. McGraw-Hill, Inc., New York. pp. 494-521, 522-539.
    5. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 427-440 and inside front cover.
    6. Runnegar, B. (1992). Evolution of the earliest animals. Major Events in the History of Life. J. W. Schopf (ed.). Jones and Bartlett Publishers, Boston. pp. 65-93.
    7. Wilson, E.O. (1992). The Diversity of Life. W.W. Norton & Co., New York.