Despite radical differences in what limbs do and what they look like,

Despite radical differences in what limbs do and what they look like, the underlying blueprint of all limbs in land based animals, whether those limbs are wings in birds, flippers in penguins, or hands in humans, is the same - one bone, the humerus in the arm or the femur in the leg, articulates with two bones, which attach to a series of small blobs, which connect with the fingers or toes. Want to make a bat wing? Make the fingers really long. Want to make a horse? Elongate the middle fingers and toes and lose the outer ones. The differences between creatures lie in differences in the shapes and sizes of the bones and the numbers of blobs, fingers, and toes.

In the 1950s and 1960s a number of biologists, including Edgar Zwilling and John Saunders, did extraordinarily creative experiments on chicken eggs to understand how skeletal structure of limbs forms and uncovered some of the key mechanisms that build limbs that have the same architecture but are as different as bird wings, lizards webbings, and human hands . They discovered that two little patches of tissue essentially control the development of the pattern of bones inside limbs. These patches of tissue were named the zone of polarizing activity (ZPA). The cells in the ZPA made special molecules that then spread across the limb to instruct cells to make femur, articulates, and connecting toes. The concentration of these molecules was the important factor and decided the length of femur, articulates, connecting toes, and even the length of individual toe fingers. Later experiments with other animals such as the bat, frog, etc. proved that the mechanism to form limbs remained the same - formation of limbs in every creature was controlled by the ZPA – just the concentration of these special molecules varied corresponding to the desired structure.

The reason for this inherent commonality in architecture still remained a conundrum for decades. Why did nature not develop architectures better optimized to the functional needs of various organisms? The discovery of Tiktaalik, a transition between non-tetrapod vertebrates ("fish") to early tetrapods solved this mystery, providing evidence that all land based creatures that have limbs, hands, etc. share a common ancestor. The fact that our DNA and that of all land based animals can be traced back to Tiktaalik further provided evidence that all appendages, whether they are hands or limbs, are built by similar kinds of genes and that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in development of Tiktaalik’s limbs, in new ways to make wings in birds or various sized limbs with fingers and toes, explaining the diversity in shapes while maintaining the similarity in architecture.

1. According to the passage the mechanism that builds limbs as different as bird wings, penguin flippers, and human hands etc. is
A. controlled through the ZPA, uses the same DNA and the same genes in different ways and only differs in the concentration of special molecules.
B. controlled through the ZPA and uses the same DNA and complementary genes suited to the shape and size of the limb.
C. defined by the DNA coming from fish and the corresponding complement of genes activated as per the needs of the shape, size and strength of the limb.
D. controlled through the ZPA in most tetrapods with some exceptions and uses DNA from corresponding ancestral heritage of tetrapods with suitable genes turned on.
E. not controlled through ZPA but differs from one vertebrate to another.



2. In the context of the passage, why was the discovery of the Tiktaalik important?
A. It proved beyond doubt and without the need of experimentation the commonality of skeletal architectures of all land based organisms.
B. It proved the reason why the architecture of limbs in various organisms was optimized for the organism’s corresponding functional needs.
C. It provided the reason for commonality of skeletal architecture described earlier in the passage.
D. It traced our ancestral heritage to the fish, providing the reason why we share majority of our DNA with aquatic animals.
E. It showed how ancient genes could be used in new ways for structures as diverse as wings in birds and fins in sharks.


3. From the passage, what can be inferred about the architecture of limbs?
A. All land-based creatures that have limbs with similar architecture are also structurally similar.
B. It is different in land-based animals due to the difference in expression of the genes.
C. It evolved since the time of Tiktaalik to suit the functional needs of the land-based animals.
D. It may have been possible to develop an architecture better optimized for the functional needs of various organisms had nature not adhered to the common architectural scheme.
E. It is defined by the concentration of special molecules present in the cells of ZPA.



4. The primary purpose of the passage is
A. To explain the reasons for the similarity in the architecture of the limbs of all land-based organisms.
B. To evaluate the implications of the experiments of Edgar Zwilling and John Saunders.
C. To trace the importance of the discovery of Tiktaalik.
D. To describe the role a common DNA plays in the unified architecture of the limbs of all land-based organisms.
E. To deduce how limbs of various sizes and shapes are built by the ZPA tissue.