Taxonomy

All creatures are classified in a hierarchical system of taxonomic classes based on the number of similarities one organism shares with another. The classes from broadest to most specific are: kingdom, phylum, class, order, family, genus, species. Each penguins species has features that make them unique from one another, but what they have in common is that they are members of the family spheniscidae.

Penguins as Birds:
Birds, as a whole class, evolved from a reptile ancestor about 150-200 million years ago during the Mesozoic era when dinosaurs still roamed the Earth. Since this period, a total of some 150,000 bird species are estimated to have inhabited the Earth. For penguins, the taxonomic classification looks like this:

Kingdom: Animalia (animals)
Phylum: Chordata (animals with backbones)
Class: Aves (birds)
Order: Sphenisciformes
Family: Spheniscidae

Penguins as Individual Species:
Penguins today are more similar to each other than to penguins of the past. Based on skeletal studies, the six different genera of penguins can be grouped in the following way: Megadyptes and Eudyptes, Spheniscus and Eudyptula, and Pygoscelis, and Aptenodytes. Although these groupings have been determined, it is not known how they evolved from one another. The main differences between the 17 penguin species is size, physical appearance, and behavior.

Fossil Classification:
When scientists study fossil records to determine what birds were the ancestors of modern penguins they use an analysis called cladistics, which looks for shared derived characteristics between fossils and modern species. Associated fossils provide clues about the age of penguins as well as the type of enviroment they lived in.

The Oldest Fossils:
Using fossils, penguins are easy to recognize due to their dense tarsometatarsus (foot) and humerus (arm) bones. Penguin skeletons date back to 55 million years ago and in 2001, the oldest fossilized penguin bones were found in New Zealand dating to almost 60 million years ago. This New Zealand penguin was 1.5-1.7 meters in height and probably weighed around 60 kg. In 2003, the earliest known South America penguin fossil was discovered in Tierra del Fuego, Argentina, revealing that penguins were in South America 40 million years ago, which is 20 million years earlier than most scientists had originally thought. Based on the age of penguin fossils and other evidence, it is clear that penguins evolved from a flying ancestor.

Pre-historic world:
Penguins evolved during the late Palaeocene and early Eocene around 60-50 million years ago and during this time, the Antarctic was covered with temperate rain forests (similar to present day New Zealand) and surrounding water temperatures were 15°C or higher. The world was very different and Australia, Antarctica, and South America were linked together in a single landmass. Antarctica was not frozen while penguins were evolving, so this suggests that the absence of fossilized penguins from Antarctica is due to a lack of ice-free sampling sites, rather than indicating that no penguins lived on Antarctica during their evolution. A circumpolar ocean did not form until 25 million years ago when South America separated from Antarctica and this event was accompanied by ice formation on Antarctica and a drop in sea surface temperature. During this climate change, two new penguin genera formed as penguins living at the Anatarctic coast adapted to life in the new, colder climate.

DNA and Behavior Classification:
Instead of focusing just on the morphology, or the ways that bones are shaped, penguin ancestory can also be determined based on DNA and behavioral similarities. This approach is helpful because scientists know little about penguin ancestors, but can compare modern penguins to each other and other living birds.

DNA Classification:
Sophisticated DNA-hybridization techniques allow scientists to determine how similar penguins are to other bird species based on the number of DNA sites they have in common. The process splits the DNA helix of two bird species and puts one strand from each bird species together to see if the two different strands will fit together. The degree to which the strands bond reveals how closely related the two bird species are. The more recently the two birds shared a common ancestor, the more matches the hybrid DNA strand will have.

A biochemical classification of birds produces a different picture of bird relationships compared to the more traditional morphological classification. Although both methods show that penguins are most closely related to loons, petrels, and albatrosses, the biochemical classification also shows that penguins are related to birds like flamingos, herons, and tropicbirds. There is controversy surrounding the use of biochemical data in classifying birds, so until the science is further perfected, the morphological classification system for birds is preferred.

Behavior Classification:
Behavior may also provide clues as to how penguin species are related, but caution must be used since behavior is more closely tied to the environment than to ancestory. One aspect of behavior is related to nest building. Emperor penguins carry their eggs on their feet because there are no stones or vegetation available on the frozen sea ice. King penguins do the same, but they breed in the sub-Antarctic and have stones and vegetation available to them. Presumably, the ancestors of King penguins were similar to Emperor penguins and the egg-holding behavior is a result of their common ancestry. This is evident by the fact that they are both members of the same Aptenodytes genus.

Differences between genera can also reveal how long each penguin species has existed. The Eudyptula genus, for example, contains only one species, the Little penguin. The Little penguin is not only the smallest penguin, but is nocturnal, swims slowest, cannot dive very deep, and does not porpoise. These more primitive characteristics cause scientists to classify it as the oldest of the modern penguin species.