The Origin and Evolution of Vertebrates

Identify four derived characters that all chordates have at some point during their life.

You are a chordate, yet you lack most of the main derived characters of chordates. Explain.

VISUAL SKILLS Based on the phylogenetic tree diagram in Figure 34.2, predict which vertebrate groups should have lungs or lung derivatives. Explain.

How are differences in the anatomy of lampreys and conodonts reflected in each animal's feeding method?

In several different animal lineages, organisms with a head first appeared around 530 million years ago. Does this finding constitute proof that having a head is favored by natural selection? Explain.

WHAT IF? Suggest key roles that mineralized bone might have played in early vertebrates.

What derived characters do sharks and tuna share? What features distinguish tuna from sharks?

Redraw Figure 34.2 to show four lineages: cyclostomes, lancelets, gnathostomes, and tunicates. Label the vertebrate common ancestor and circle the lineage that includes humans.

WHAT IF? Imagine that we could replay the history of life. Is it possible that a group of vertebrates that colonized land could have arisen from aquatic gnathostomes other than the lobe-fins? Explain.

Describe key adaptations of aquatic gnathostomes.

Describe the origin of tetrapods and identify some of their key derived traits.

Some amphibians never leave the water, whereas others can survive in relatively dry terrestrial environments. Contrast the adaptations that facilitate these two lifestyles.

WHAT IF? Scientists think that amphibian populations may provide an early warning system of environmental problems. What features of amphibians might make them particularly sensitive to environmental problems?

Describe three key amniote adaptations for life on land.

Are snakes tetrapods? Explain.

Identify four avian adaptations for flight

Based on the phylogeny shown in Figure 34.25, identify the sister group for (a) reptiles; (b) squamates; and (c) the clade that includes crocodilians and birds.

Contrast monotremes, marsupials, and eutheriansin terms of how they bear young.

Identify at least five derived traits of primates.

MAKE CONNECTIONS Develop a hypothesis to explain why the diversity of mammals increased in the Cenozoic. Your explanation should consider mammalian adaptations as well as factors such as mass extinctions and continental drift (review Concept 25.4).

Calculate the means ( $\overline{x}$and $\overline{y}$ ) from the n= 8 data points in the table. Next, fill in the ( $x_i - \overline{x}$) and ($y_i - \overline{y}$ ) columns in the data table, and use those results to calculate the standard deviations s_x and s_y.

As described in the Scientific Skills Exercise for Chapter 32, the formula for a correlation coefficient is  $r=\frac{\frac{1}{n-1}\sum (x_i-\overline{x)}(y_i-\overline{y})}{s_x{s}_y}$

Fill in the column in the data table for the product ( $x_i - \overline{x}$)×( $y_i - \overline{y}$). Use these values and the standard deviations calculated in question 1 to calculate the correlation coefficient r between the brain volume of hominin species (y) and the ages of those species (x).

Based on the value of r that you calculated in question 2, describe in words the correlation between mean brain volume of hominin species and the mean age of the species.

(a) Use your calculated value of r to calculate the slope (m) and the y-intercept (b) of a regression line for this data set. (b) Graph the regression line for the mean brain volume of hominin species versus the mean age of the species. Be careful to select and label your axes correctly. (c) Plot the data from the table on the same graph that shows the regression line. Does the regression line appear to provide a reasonable fit to the data?

The equation for a regression line can be used to calculate the value of y expected for any particular value of x. For example, suppose that a linear regression indicated that m = 2 and b = 4. In this case, when x = 5. We expect that y= 2x + 4 = (2 × 5) + 4 = 14. Based on the values of m and b that you determined in question 4, use this approach to determine the expected mean brain volume for a hominin that lived 4 million years ago (that is, x = -4)

The slope of a line can be defined as $m=\frac{y_2-y_1}{x_2-x_1}$ , where (x₁, y₁) and (x₂, y₂) are the co-ordinates of two points on the line. As such, the slope represents the ratio of the rise of a line (how much the line rises vertically) to the run of the line (how much the line changes horizontally). Use the definition of the slope to estimate how long it took for mean brain volume to increase by 100 cm³ over the course of hominin evolution.

Identify some characters that distinguish hominins from other apes.

Provide an example in which different features of organisms in the hominin evolutionary lineage evolved at different rates.

Some genetic studies suggest that the most recent common ancestor of Homo sapiens that lived outside of Africa left Africa about 50,000 years ago. Compare this date with the dates of fossils given in the text. Can both the genetic results and the dates ascribed to the fossils be correct? Explain.

Vertebrates and tunicates share

(A) jaws adapted for feeding.

(B) a high degree of cephalization.

(C) an endoskeleton that includes a skull.

(D) a notochord and a dorsal, hollow nerve cord.

Living vertebrates can be divided into two major clades. Select the appropriate pair.

(A) the chordates and the tetrapods

(B) the urochordates and the cephalochordates

(C) the cyclostomes and the gnathostomes

(D) the marsupials and the eutherians

Unlike eutherians, both monotremes and marsupials

(A) lack nipples.

(B) have some embryonic development outside the uterus.

(C) lay eggs.

(D) are found in Australia and Africa.

Which clade does not include humans?

(A) synapsids

(B) lobe-fins

(C) diapsids

(D) osteichthyans

As hominins diverged from other primates, which of the following appeared first?

(A) reduced jawbones

(B) an enlarged brain

(C) the making of stone tools

(D) bipedal locomotion

Which of the following could be considered the most recent common ancestor of living tetrapods?

(A) a sturdy-finned, shallow-water lobe-fin whose appendages had skeletal supports similar to those of terrestrial vertebrates

(B) an armored, jawed placoderm with two pairs of appendages

(C) an early ray-finned fish that developed bony skeletal supports in its paired fins

(D) a salamander that had legs supported by a bony skeleton but moved with the side-to-side bending typical of fishes

EVOLUTION CONNECTION Living members of a vertebrate lineage can be very different from early members of the lineage, and evolutionary reversals (character losses) are common. Give examples that illustrate these observations and explain their evolutionary causes.

WRITE ABOUT A THEME: ORGANIZATION Early tetrapods had a sprawling gait (like that of a lizard): As the right front foot moved forward, the body twisted to the left and the left rib cage and lung were compressed; the reverse occurred with the next step. Normal breathing, in which both lungs expand equally with each breath, was hindered during walking, and prevented

during running. In a short essay (100–150 words), explain how the origin of organisms such as dinosaurs, whose gait allowed them to move without compressing their lungs, could have led to emergent properties.

This animal is a vertebrate with hair. What can you infer about its phylogeny? Identify as many key derived characters as you can that distinguish this animal from invertebrate chordates.

SCIENTIFIC INQUIRY • DRAW IT As a consequence of size alone, larger organisms tend to have larger brains than smaller organisms. However, some organisms have brains that are considerably larger than expected for their size. There are high energetic costs associated with the development and maintenance of brains that are large relative to body size.

(a) The fossil record documents trends in which brains that are large relative to body size evolved in certain lineages, including hominins. In such lineages, what can you infer about the costs and benefits of large brains?

(b) Hypothesize how natural selection might favor the evolution of large brains despite their high maintenance costs.

(c) Data for 14 bird species are listed below. Graph the data, placing deviation from expected brain size on the x-axis and mortality rate on the y-axis. What can you conclude about the relationship between brain size and mortality?