Week 4 Archaeology
Weekly Topics and subtopics
Evolutionary lines, trees, bushes, and species
Species Concepts and Hominins
Species seem like natural categories of life because in the courses of our normal daily activities we do not see enough to make us wonder about them, but species enigmas are everywhere. Did you know that the local sea gulls and salamanders do not form the same kind of 'species' that we are used to?
A classic variety of complicated species is called a 'ring species.' Sometimes populations of a species are distributed in the shape of a ring around an area that is inhospitable or has no resources. Sometimes there is a break in the ring! There is a genus of salamander from the Coast Range and the Sierra Nevada called Ensatina that has variants with continuous gene flow starting around Santa Barbara, going up the coast range to Humboldt County, winding across the Cascade Range, and back down the Sierra Nevadas. It is like one big continuous species, except in the Tehachapi Mountains. It the Tehachapi area the Santa Barbara forms and the southern Sierra forms DO NOT INTERBREED.
Another classic example of ring species is the Larus gulls' circumpolar species 'ring'. The range of these gulls forms a ring around the North Pole, which is not normally crossed by individual gulls.
The European Herring Gull (L. argentatus argenteus), which lives primarily in Great Britain and Ireland, can hybridize with the American Herring Gull (L. smithsonianus, living in North America), which can also hybridize with the Vega or East Siberian Herring Gull (L. vegae), the western subspecies of which, Birula's Gull (L. vegae birulai), can hybridize with Heuglin's gull (L. heuglini), which in turn can hybridize with the Siberian Lesser Black-backed Gull (L. fuscus). All four of these live across the north of Siberia. The last is the eastern representative of the Lesser Black-backed Gulls back in north-western Europe, including Great Britain.
The Lesser Black-backed Gulls and Herring Gulls are different enough that they don't interbreed; thus the group of gulls forms a continuum except where the two lineages meet in Europe.
As recent genetic studies show, this example is far more complicated than the salamander example, and there are several other taxonomically unclear examples that belong in the same sea gull superspecies complex, like the Yellow-legged Gull (L. michahellis), Glaucous Gull (L. hyperboreus), and Caspian Gull (L. cachinnans).
So species are complicated things in the modern world. But what about fossils? Most Paleontologists do not think of species the same way that zoologists working on living animals think of them. Paleontologists try to understand the way that species operate in nature to inform the way that they interpret the fossil record, but they do not have many of the vital interpretive tools of zoologists. The do not have soft tissue, they do not have DNA (in most cases), and they do not have behavior. For example, the only reason that we know that the species described above are 'ring species' is because we know about mating behavior.
It turns out that the different species of the human fossil record are more typological. There is not very much evidence about actual reproductive boundaries preserved in the fossil record, and we are resigned to organizing groups into rough categories of shape that occur in specific places at particular times.
Unfortunately, the intense world focus on human origins makes for conceptual problems when it comes to species. Many pop-scientists portray hominid species as if they were the same thing as modern species, neglecting to inform their audience that this is only true in the metaphoric sense. It is better to think of hominid species as functional categories of our ancestors that utilize shape, space, and time to construct useful categories.
One must be careful. The study of human origins is hugely popular and very international. There are many voices speaking at once, and there are many small scenes of paleoanthropologists around the world. There are also a lot of people who would like to oversimplify. Have you ever seen one of the evolutionary trees with the hominid species as points and branches going between them? The diagram below is from the American Museum of Natural History. The general public appreciates the simple picture of human origins diagrams like this provide, but they are mistaken to think of these as species in a modern sense. Each has different parameters that define it.
For the sake of this class it is better to memorize the hominid genera and to imagine them as grades, stages of evolution that can be characterized by large scale trends in shape and behavior. The three hominid genera are Ardipithecus, Australopithecus , and Homo. Within each of these genera many species have been recognized, and we will be looking at many of them, but start with the genera. It is much more straightforward. Click to the next concept to see the major evoutionary grades of hominids.
A classic variety of complicated species is called a 'ring species.' Sometimes populations of a species are distributed in the shape of a ring around an area that is inhospitable or has no resources. Sometimes there is a break in the ring! There is a genus of salamander from the Coast Range and the Sierra Nevada called Ensatina that has variants with continuous gene flow starting around Santa Barbara, going up the coast range to Humboldt County, winding across the Cascade Range, and back down the Sierra Nevadas. It is like one big continuous species, except in the Tehachapi Mountains. It the Tehachapi area the Santa Barbara forms and the southern Sierra forms DO NOT INTERBREED.
Another classic example of ring species is the Larus gulls' circumpolar species 'ring'. The range of these gulls forms a ring around the North Pole, which is not normally crossed by individual gulls.
The European Herring Gull (L. argentatus argenteus), which lives primarily in Great Britain and Ireland, can hybridize with the American Herring Gull (L. smithsonianus, living in North America), which can also hybridize with the Vega or East Siberian Herring Gull (L. vegae), the western subspecies of which, Birula's Gull (L. vegae birulai), can hybridize with Heuglin's gull (L. heuglini), which in turn can hybridize with the Siberian Lesser Black-backed Gull (L. fuscus). All four of these live across the north of Siberia. The last is the eastern representative of the Lesser Black-backed Gulls back in north-western Europe, including Great Britain.
The Lesser Black-backed Gulls and Herring Gulls are different enough that they don't interbreed; thus the group of gulls forms a continuum except where the two lineages meet in Europe.
As recent genetic studies show, this example is far more complicated than the salamander example, and there are several other taxonomically unclear examples that belong in the same sea gull superspecies complex, like the Yellow-legged Gull (L. michahellis), Glaucous Gull (L. hyperboreus), and Caspian Gull (L. cachinnans).
So species are complicated things in the modern world. But what about fossils? Most Paleontologists do not think of species the same way that zoologists working on living animals think of them. Paleontologists try to understand the way that species operate in nature to inform the way that they interpret the fossil record, but they do not have many of the vital interpretive tools of zoologists. The do not have soft tissue, they do not have DNA (in most cases), and they do not have behavior. For example, the only reason that we know that the species described above are 'ring species' is because we know about mating behavior.
It turns out that the different species of the human fossil record are more typological. There is not very much evidence about actual reproductive boundaries preserved in the fossil record, and we are resigned to organizing groups into rough categories of shape that occur in specific places at particular times.
Unfortunately, the intense world focus on human origins makes for conceptual problems when it comes to species. Many pop-scientists portray hominid species as if they were the same thing as modern species, neglecting to inform their audience that this is only true in the metaphoric sense. It is better to think of hominid species as functional categories of our ancestors that utilize shape, space, and time to construct useful categories.
One must be careful. The study of human origins is hugely popular and very international. There are many voices speaking at once, and there are many small scenes of paleoanthropologists around the world. There are also a lot of people who would like to oversimplify. Have you ever seen one of the evolutionary trees with the hominid species as points and branches going between them? The diagram below is from the American Museum of Natural History. The general public appreciates the simple picture of human origins diagrams like this provide, but they are mistaken to think of these as species in a modern sense. Each has different parameters that define it.
For the sake of this class it is better to memorize the hominid genera and to imagine them as grades, stages of evolution that can be characterized by large scale trends in shape and behavior. The three hominid genera are Ardipithecus, Australopithecus , and Homo. Within each of these genera many species have been recognized, and we will be looking at many of them, but start with the genera. It is much more straightforward. Click to the next concept to see the major evoutionary grades of hominids.
More information