Step-by-Step: Major Evolutionary Transitions Quiz

Answer: Both DNA and proteins require the other for their own synthesis.

Although proteins and DNA are perfectly suited for their specific functions within living organisms, there is an ongoing debate as to which evolved first. The problem is that DNA stores within itself the framework for constructing proteins and proteins are needed to "read" the DNA in order to initially make proteins.

Therefore, since DNA makes the proteins that make DNA, the development of the cell seems impossible unless all of the parts are already present in their proper places. Another cellular compound, RNA, has been postulated as the answer to this paradox. RNA is a similar compound to DNA and can perform both of the functions of DNA and proteins.

The first cells may have used RNA in these dual roles. As for the reason that the world didn't remain RNA-based, the saying "jack-of-all-trades, master of none" applies. RNA wasn't as good as DNA at storing hereditary data or as efficient as proteins and was replaced; however, its versatility ensured that it was still retained for other cellular processes.

Answer: Molecular evidence may indicate a much more ancient origin for eukaryotes than previously thought.

Although the conception that evolution necessarily must progress from simple to complex is not true, for most of scientific history this idea has been applied to the transition from one-celled to many-celled organisms. Using recent molecular DNA evidence, some scientists are questioning the long-held belief that prokaryotes came first and were the building blocks for eukaryotes. ESPs (eukaryotic signature proteins) code for innovations and inventions that are unique to eukaryotes and have no analog in prokaryotes. If eukaryotes evolved from prokaryotes, there should be some counterpart to these proteins in the prokaryotic genome. Additionally, most of the features of the eukaryotic cell appear to have evolved under anaerobic (low oxygen) conditions. Most estimates of the earliest appearance of the eukaryotes have them appearing after photosynthetic organisms had increased the oxygen content of the atmosphere. If they formed before this event, then their age could be pushed back to ~3 billion years ago (right up there with the prokaryotes). Our origins, and a critical transition in the history of life, could be much more ancient than we thought.

Answer: Invertebrate - Vertebrate

The invertebrate - vertebrate transition happened over 500 million years ago. The fossil record for this transition is surprisingly detailed thanks to many amazing discoveries made in Yunnan Province, China. There is a clear progression from invertebrate acorn worms to early chordates (Amphioxus and Myllokunmingia) to proto-vertebrates Haikouella and Haikouichthys.

While primitive chordates were around at this point (chordates split off from the ancestors of starfish and sea urchins earlier), the vertebrate lineage ushered in internal bony skeletons and the ability to evolve into larger, more complex forms.

This breakthrough allowed for a major diversification of life into previously under-utilized niches and ecosystems.

Answer: Jaws

The transition between jawless and jawed fishes occurred around 460 million years ago. Thelodonts and Osteostracans are the groups that ushered in this major shift. Most of the jawless fishes have since died out but the jawed gnathostomes have diversified and include the bony and cartilaginous fishes as well as amphibians, reptiles, birds, and mammals.

The jaw may have evolved from the first pair of gill arches in jawless fishes and became an important feeding structure. It allowed for the consumption of prey larger than the animal itself and it provided attachment sites for another feeding structure: teeth.

Answer: Lobe-finned fishes (Sarcopterygii)

It was not an easy step but it was one that could be accomplished through successive modifications of already existing structures. The lobed fins of these fish contained the bones and structures that evolved into the shoulder, arm, wrist, and hand as well as the femur, leg, ankle, and foot of tetrapods. Eusthenopteron and the more tetrapod-like fish Panderichthys have a variety of features shared only between the lobe-finned fishes and tetrapods.

These features include a flatter head with eyes on top, internal nostrils, the arm and leg precursors, and lungs.

These adaptations allowed these fish to spend brief periods of time in shallow, muddy shoals in order to exploit prey items or escape predation. The first shaky steps from sea to land had been taken, but there was more that needed to be done before fully terrestrial animals colonized the land.

Answer: Tiktaalik

Tiktaalik roseae demonstrates an intermediate phase in the takeover of the land by lobe-finned fishes. Eusthenopteron and Panderichthys were pelagic despite some occasional forays onto land. Ichthyostega and Tulerpeton are more developed tetrapods with limbs and very specific land adaptations. Tiktaalik slots neatly into place between these two groups.

It combines both fish-like and tetrapod-like features. It has distinct arm structures (including for the first time a wrist), a fully flat head with eyes on top (as opposed to a round head with eyes on the side), and a neck.

These may not sound like much but they were crucial to tetrapod development. The arm and wrist were weight-bearing, allowing Tiktaalik to move its body about in a more controlled fashion on land.

The neck allowed it to move its head independently of its body; fish don't have necks and must move their whole body to look around. It also had primitive lungs for air-breathing on land and spiracles that were precursors to the tetrapod ear.

Answer: The Permian-Triassic mass extinction.

The importance of this change of power between the synapsids and the archosaurs is due to the fact that the synapsids were the group from which mammals evolved. The fossil record shows a clear progression from pelycosaurs (like Dimetrodon), to the terrifyingly predatory Gorgonopsids, to the early mammals.

These early mammals were ravaged by the Permo-Triassic extinction event. This mass extinction, also called The Great Dying, was the most devastating in Earth's history. Over 95% of marine species and 70% of terrestrial vertebrates perished for reasons that are still unknown.

The ancestors of mammals were poised on the cusp of land supremacy before random chance took the opportunity away. Archosaurs, the ancestors of the dinosaurs, rapidly spread into the niches vacated by the proto-mammals and then dominated for almost 200 million years.

The transition from tetrapods to reptiles (and from them to dinosaurs) and to mammals would have profound impacts on the evolutionary history of life on Earth.

Answer: Birds

The first reptile-bird fossil discovered was Archaeopteryx lithographica in 1861 in the famous Solnhofen quarry in Germany. This animal was a perfect transition specimen because it combined both dinosaurian and avian traits in one animal. It had teeth, hind limbs, and a tail like a dinosaur, but it also had avian forelimbs (wings) and, perhaps the most important feature of all, feathers.

The fossil record was further fleshed out in the 1990s with the discovery of many new "dinobirds" from Liaoning Province, China.

These included Protoarchaeopteryx (with the beginning of wing formation), Confuciusornis (with a toothless beak and a fused tail), and Sinornis (with an even shorter tail, an opposable toe for perching, and a large breastbone). Equally remarkable were the discoveries in China of non-avian feathered dinosaurs. Creatures like Sinosauropteryx prima and Beipiaosaurus inexpectus were covered in a coat of feathers.

This indicated that feathers evolved earlier than thought and first formed more for thermoregulatory purposes and were only later co-opted for flight.

Answer: Some mammals returned to an aquatic lifestyle from the land.

The reverse transition from the land back to the sea has happened several times. The first time was the return of several groups of reptiles to the ocean (these became the ichthyosaurs, mosasaurs, and plesiosaurs). When these large predators went extinct at the end of the Cretaceous, a number of mammalian groups filled in the vacated niches.

The pinnipeds (walruses, seals, and sea lions) all have detailed transitional forms demonstrating their evolution from bear-like ancestors to intermediate aquatic forms (Enaliarctos) to fully marine-adapted specimens.

The dugongs and manatees (Sirenians) also show evolution from early elephant-like forms to their fully aquatic modern descendants. One of these forms, Pezosiren, discovered in 2001 is essentially a walking sirenian.

The cetaceans (whales and dolphins) have one of the best transitional fossil records. Specimens range from the semi-aquatic Pakicetus and Ambulocetus to more marine forms with front limbs modified into flippers and reduced (but still visible) hind limbs such as Gaviacetus and the 60-ft Basilosaurus.

The modern cetaceans are now fully aquatic but still retain small, useless hind limbs buried in muscle tissue as vestiges of their evolution from land-dwelling mammals.

Answer: Moving other hominids (chimps and gorillas) into the human genus Homo.

This bickering between taxonomists about human classification may seem unimportant but it has huge ramifications. For our entire history, we as a species have had an understandably anthropocentric view of the world. Humans were viewed as special and therefore deserved special considerations and rules (and exemptions from standard rules).

The main reason for the classification of humans and chimps into different genera was because of this special view. Genetically, humans and chimps (and to a lesser extent gorillas) are so close that, according to the rules of classification, they should be in the same genus.

Some taxonomists argue for either moving chimps and gorillas into our genus Homo or moving humans into the chimp genus Pan. If either of these moves occurs, it could undermine the belief in our specialness.

This will have profound philosophical and religious impacts. The chimpanzee-human transition (whatever its classification) is believed to have occurred over 5-7 MYA (according to molecular and genetic analysis).

The fossils of Sahelanthropus tchadensis and Orrorin tugenensis both originate around this split and show that even this early in our evolution we had upright posture and the beginning of a large brain. This last transition is the most significant for us but it is just one of many in the long history of life.