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Equability in the late Pleistocene

by Larry D. Martin and Jean (Bright) Martin

The University of Kansas, Lawrence, Kansas


The traditional idea that continental glaciation was accompanied by a harsh, cold environment does not seem to apply to North America. Abundant evidence now supports a glacial climate of cooler summers and warmer winters over most of nonglaciated North America. The low seasonality of this climate permitted plants and animals to have broader ranges than they have today, and these broader ranges resulted in overlaps of species ranges that do not now occur. This created very complex Pleistocene community structures in North America that do not have close modern analogs. The development of highly seasonal climates during the Holocene resulted in the destruction of these Pleistocene communities and the development of the modern biomes. Megafauna extinction and changes in human cultural evolution were closely associated with this climatic change.


Our understanding of the Pleistocene has undergone many changes in the last decade. For instance, the idea that there were more glacial advances in North America than the traditional four (Nebraskan, Kansan, Illinoian, and Wisconsinan) was quite radical in 1966 (Dort, 1966), but is now widely accepted (Boellstorff, 1978). Previously interglacials were accepted as having been many times longer in duration than were glacials, but the new deep-sea evidence (Hays et al., 1976) shows that the opposite was true with interglacials about 10,000 yrs in length and glacials nearer 100,000. Also our present climate has been generally accepted to be typical for an interglacial, but evidence is accumulating that the Holocene climate is unique for the Quaternary.

The most useful change in our way of viewing the Ice Age has had to do with the concept of climatic equability. Throughout continental interiors the modern world is dominated by climates with high seasonal variability. In order to exist in these climates, organisms must maintain a suite of conflicting adaptations. They must survive extended periods of both arctic and tropical conditions, utilize seasonal rains, and survive in seasonal deserts. In order to do this, they commonly adjust their reproductive timing to a narrow interval of favorable climate and thereby become vulnerable to permutations of this interval. Organisms that have solved these problems are now present in enormous numbers and cover vast areas of the continental interiors. They exist in biotic systems that are quite different from those of the Pleistocene. In the Pleistocene the differences between seasons must have been greatly reduced; thus a general theory of climatic equability has been developing during the last three decades (Hibbard, 1960; Taylor, 1965; Martin and Neuner, 1978; Graham and Lundelius, 1984).

Abundant evidence now supports a glacial climate of cooler summers and warmer winters over most of nonglaciated North America (Lundelius, 1967; Slaughter, 1975). The low seasonality of this climate permitted plants and animals to have broader ranges than they have today, and these broader ranges resulted in overlaps of species ranges that do not now occur. This created very complex Pleistocene community structures in North America that do not have close modern analogs; this was true for plants (Van Devender and Mead, 1976) as well as for animals.

Almost all of unglaciated North America was more forested during the Wisconsinan, and no extensive steppes or deserts existed. Tundra was restricted to a very narrow band along the margins of the continental glaciers and to areas of high altitude. Beringia and a few places south of the continental ice seemed to have been occupied by steppe tundra.

We know little about the people who occupied these special Pleistocene environments, and indeed some (Martin, 1973) think that people arrived in North America after the beginning of the collapse of these environments. They would accept a duration of man in North America of only about 12,000 yrs. We believe that this viewpoint can no longer be supported against numerous reports of sites dated at ages greater than 12,000 yrs (MacNeish, 1976). The archeological data suggest at least 20,000 yrs for the duration of man in the New World.

Distinctive floral regions during the Wisconsinan had distinctive faunas (Martin and Neuner, 1978). The western region of North America, south of the ice, was occupied by a montane conifer parkland that contained the Camelops faunal province. The Ovibos faunal province falls along the edge of the continental ice and in high-altitude sites in the mountains. The northeastern United States was occupied by a spruce-forest taiga that contained the Symbos-Cervalces faunal province. The southeastern region was occupied by a deciduous forest that contained the Chlamythere-Glyptodont faunal province.

Beginning about 12,000 yrs ago, the complex Pleistocene communities began to break up and by 8,000 yrs ago, we have essentially the modern distribution of plants and animals. The collapse of the Ice Age floral communities was accompanied by the extinction of over half of the species of North American large mammals.

Disharmonius associations

Almost all Pleistocene biota contain some associations of organisms that cannot be found living together today and commonly include taxa that are presently allopatric by many hundreds or thousands of miles. Semken (1974) termed these disharmonius associations. The implication is that the modern associations are in "harmony" with known requirements of the organisms, and modern associations could be called "harmonius associations." This terminology carries a false implication that modern associations are normal while Pleistocene ones are not. The reverse is more likely, but the term itself does call attention to an interesting aspect of Pleistocene communities.

The limits of the distribution of organisms are fixed by a wide variety of factors, but the controlling factors may only be limiting for short periods of time. In other words, seasonal averages do not limit the range of organisms so much as do periods of extremes. The reduction of seasonal extremes is the one overall factor that could cause animals and plants of both southern and northern environments to simultaneously expand their ranges and create overlaps. This results in the creation of greater local diversities and the development of new coevolved systems and more complex community structures.

Disharmonius assemblages seem to be ubiquitous throughout the Pleistocene and to occur in both putative glacial and interglacial assemblages. Their presence is one of the most distinctive contrasts between Pleistocene biota and those from the Holocene. As such, they demarcate the Holocene very dramatically from the Pleistocene and are part of the reason that Martin and Neuner (1978) argued that the Pleistocene had really ended.

Winter warmth

Our understanding of past organisms and environments is rooted in our understanding of modern analogs, and there is a tendency to read too much of present conditions into the fossil record. Such an error seems likely to have largely dominated our interpretation of Cenozoic climates in North America. In spite of the occurrence of clearly tropical animals such as tapirs and crocodilians, researchers often interpret the Tertiary environments of central North America as the same treeless steppes with cold snowy winters that exist in that region today. The Pleistocene with its vast continental ice sheet was taken to be evidence of harsh climates as severe or more severe than those existing in the high arctic today. Strangely the fossil record has never lent support to such interpretations, and many of them must now be abandoned.

Hibbard (1960) probably deserves the major credit for our reassessment of late Cenozoic climatic conditions. Hibbard pointed out that modern large tortoises (Geochelone) cannot tolerate extended periods of below-freezing temperatures and that the common occurrence of the extinct giant tortoise, Hesperotestudo, in North American late Cenozoic faunas also must indicate the absence of long periods of such temperatures. During the Pliocene (Blancan), this frost-free line may have extended at least as far north as north-central Nebraska (Sand Draw local fauna). Similar tortoises continue into the Irvingtonian and Rancholabrean, but their range becomes progressively more southerly so that the northernmost late Irvingtonian (Sheridanian) Hesperotestudo is from the Angus local fauna in south-central Nebraska and the northernmost Rancholabrean record is from central Oklahoma.

Some researchers (including Hibbard, 1960) have tried to restrict the Pleistocene giant tortoise records to the interglacials which were considered exceptionally warm periods. Most Pleistocene faunas are mixtures of animals that are both at present more southerly in distribution and more northerly. The northernmost Rancholabrean records of tortoises in Oklahoma are much more northern than we could at present expect these tortoises to occur, but radiocarbon dating shows their age to be well within the generally accepted period of Wisconsinan glaciation. Giant tortoises also continued in Florida until the very end of the Pleistocene and it would seem that glacial temperatures in Florida were not so low as to exclude giant tortoises until the end of the Pleistocene.

Deducing a picture of glacial winters from this information may be possible. Climates undoubtedly were severe near the continental glacial front. This must have been a region of cold-air drainage continually swept by cold dry winds. The presence of frost-wedge casts and local areas of patterned ground indicate that glacial climates along the glacial front could be as severe as those presently found in the high arctic. This area was occupied by typical arctic-tundra animals (Martin and Neuner, 1978). Distances much greater than 200 km (120 mi) from the front may have been quite different. In some ways the growth of the glacier would demand more warmth. The upper surface of the continental ice must have been a cold desert as are the modern Arctic and Antarctic ice sheets. The glaciers did most of their growing on the margins where warm moist air would contact the glacial front. Winter warmth would actually increase the amount of available moisture and, up to the limit where melting exceeded snowfall, could actually enhance the growth of the glaciers.

Summer cool

All large Pleistocene faunas contain elements that are at present restricted farther north. This is as true for the so-called interglacial faunas as it is for the glacial ones. In fact it has been extremely difficult to demonstrate that any fauna is interglacial, although many have been suggested. In virtually all cases these assignments are based on the presence or absence of only one or two taxa. For instance Cragin quarry and the closely associated Jinglebob local fauna have both been considered Sangamonian (Hibbard, 1970), but of the two, the Jinglebob local fauna contains the most warmth-adapted mammal (Rice rat, Orizomys). The Jinglebob local fauna has since been assigned to the early portion of the Wisconsinan glacial stage (Kapp, 1970).

Kansas winters are at present adequately cold for even arctic species. Colder winters offer no advantage to boreal taxa, but cooler summers do and it must surely be hot dry summers that prevent the colonization of Kansas by cool-steppe species like the ground squirrel Spermophilus elegans.

Vegetation and seasonal climates

In the tropics the reproductive activities of plants can be scattered throughout the year. This provides fruits and flowers continuously and permits specialists in these rich food sources to develop among the herbivores. In highly seasonal climates the reproductive activities of plants tend to be concentrated into relatively short periods when temperature and moisture are favorable. Herbivores must either depend on the vegetative parts of plants or remain inactive when food is not available. When food is available it may be so plentiful that only small portions of it can be utilized by any one taxon. Seed eating becomes particularly important because seeds keep for long periods past the fruiting season and constitute a more reliable food supply.

Pleistocene grasslands were largely dominated by C3 plants. C3 grasses tend to be green throughout the year and to store more of their energy in their vegetative parts, as opposed to their roots. C4 plants turn brown during the summer drought and during the winter and thus have a shorter growing season. They store more of their energy in their roots and less in the stems and leaves. C4 grasses also become green later in the spring. On the whole, they are a less consistent source of food than are the C3 plants, and the changes to C4-grass domination may have had serious consequences for grazing mammals. This effect would be intensified for groups that synchronized the birth of their offspring to the first spring growth of the C3 grasses. This would likely be weeks before the first growth of C4 grasses.

Seasonal climates would have an additional effect on vegetation. In wet climates the spread of fires is limited by abundant rainfall and in dry climates by the scarcity of plant material. Seasonal climates are often wet part of the year and then very dry for the rest of it. Under these conditions a lush growth of vegetation that later dries out and burns is possible. Under such conditions fires may be severe and become important determining factors influencing the nature of vegetation. The modern prairies need such fires for their maintenance (Wells, 1976; Wells and Hunziker, 1976).

Pleistocene extinctions

Martin and Gilbert (1978) proposed a then-novel hypothesis for the extinction of the North American megafauna. They suggested that the Pleistocene communities were not analogous to the modern ones that succeeded them so what had really happened was the extinction of entire Pleistocene biomes. They described this mechanism for extinction in the following way (Martin and Gilbert, 1978, p. 115):

It seems likely that the animals which became extinct at the end of the Pleistocene were adapted to habitats which ceased to exist in North America and that their extinction was due to habitat destruction on a massive scale. This hypothesis implies that we are presently living under the same climatic conditions which resulted in the extinction of the large Pleistocene mammals.

This theme is also supported in the same volume by Martin and Neuner (1978, p. 124):

We believe that the low seasonality of Pleistocene environments permitted the establishment of very complex communities that lack modern analogues. These communities were composed in part of animals and plants extant today but presently allopatric and by animals which are now extinct. The modern highly seasonal environment is thought by us to be unique for the Pleistocene and its establishment along with the modern pattern of floral distributions is the underlying cause of the end-Pleistocene extinction. This mechanism would be world-wide in scope and applicable to Tertiary extinctions.

Since then, Graham and Lundelius have proposed a similar scheme where they argue that the breakup of the disharmonius biotic assemblages would also destroy coevolved relationships and result in a sort of "coevolutionary disequilibrium" and extinction. Graham and Lundelius (1984, p. 243) describe how the

Environmental changes at the end of the Pleistocene caused a major biotic reorganization. Instead of simple shifts of biotic zones, individual species responded to these environmental changes in accord with their own tolerance limits. This individualistic response of each species reduced the predictability of the composition and structure of the new communities. In coevolved systems these changes would disrupt coevolutionary relationships between plants and animals, thus creating a disequilibrium in the system.

Pleistocene equability and cultural evolution

Ice Age man lived in a world very different from the barren habitat often ascribed to him. In North America he may have lived in a relatively lush environment with warm winters and cool summers. The hunting and gathering options in any given locality were better than they would be today, and the chance for success at either hunting or gathering would vary less with the seasons. The collapse of these environments between 12,000 and 10,000 yrs B.P. must have severely stressed cultures and subjected humanity to selective pressures that had either not been present or so marked since the inception of tool-using humans some two million years earlier.

In a more equable climate, the need for clothing and shelter and the probability of finding food is more nearly uniform throughout the year. With comparatively diverse hunting and gathering resources available during the entire year, utilizing a nomadic way of life and specializing on hunting would be more advantageous. Very little planning may have been necessary because the odds of success did not vary much from one season of the year to the other. Seasonal specializations would not have been so important because the development of vast migrating herds or flocks had probably not yet developed, and the growing seasons of plants may have been extended.

Beginning about 12,000 yrs ago, the climate began to change to modern highly seasonal conditions. This change resulted in our present distribution of plants and animals and probably also caused the Pleistocene megafauna extinction. The mammals that survived this extinction are without exception forms that are rare in faunas older than 12,000 yrs B.P. With the close of the Pleistocene, the resources for human exploitation that were most important during the Pleistocene became rare or absent (Martin and Martin, 1982, 1983, 1984).

The basic structural changes that took place in floral communities between 12,000 and 10,000 yrs B. P. included a general decrease in forested regions with the development of treeless steppes and deserts. This was accompanied by the extinction of a diverse large-mammal fauna and a progressive increase in abundance of certain specialized grazers (bison and pronghorn antelope). Much of the evidence for Pleistocene big-game hunters in North America is from the central Great Plains, and the Pleistocene/ Holocene transition in this area is mostly a record of a shift from hunting large extinct prey like mammoths to progressively larger mass kills of bison (Rogers and Martin, 1984). The appearance of mass kills is probably the result of two factors. One of these is the developing severity of the winter season so that winter hunting might have been curtailed and a need arose to acquire a large meat reserve. The other factor would be the development of large herds of bison on the newly developing steppes. Changes of the distribution of small vertebrates and mollusks show that the understory vegetation was greatly affected, and gathering patterns must have changed profoundly (Martin et al., 1985).

During the Pleistocene trees were much more abundant in the central Great Plains than they are in the Holocene, and many of these trees were conifers. This means that wood, bark, and resin would be readily available for the manufacture of weapons, tools, and baskets. We would expect a more wood-dominated culture with a greater reliance on fire-hardened points, bark baskets, and other wooden artifacts. A much higher percentage of the lithics from this period should be specialized tools for the working of wood. Pleistocene localities, both in North America (Rogers, 1984) and elsewhere, have often been characterized by the presence of large chopping tools. Large numbers of such tools would be required for any culture that depended much on wooden artifacts. We would suggest that the Pleistocene cultures in general are characterized by lithic assemblages composed of higher percentages and varieties of wood-working tools than are found in Holocene collections from the central Great Plains.

Storing food was a greater need as seasonal extremes made it temporally difficult to hunt or to gather. On the other hand seasonal migrations or seasonally clustered fruitings of plants may have made some resources locally abundant for short periods of time. These seasonal abundances would require planning and preparation, and selection for the development of a calendar would increase. This might also promote the banding together of groups of people in order to cooperate and to utilize more fully the seasonal resource. Once these resources were collected and stored, the ability of these people to travel would be limited by their need to remain and care for their food reserve. The very existence of large reserves of resources would also create an incentive for warfare and theft. The threat of these activities might also tend to bind people together in sedentary groups. The existence of permanent or semi-permanent base camps would seem to be a necessary precursor to the development of agricultural societies (Martin and Martin, 1983, 1984). In other words the development of highly seasonal climates would create a selection regime favoring the development of more efficient food storage, seasonal planning and calendars, cooperative seasonal acquisition, and sedentary cultures ultimately leading to agriculture. It may be no accident that cultural evolution was so much more rapid during the Holocene than in the Pleistocene and that most sedentary agricultural societies have their roots in the first half of the Holocene.


For the bulk of the last two million years, plants and animals have been evolving in North America under conditions of lesser seasonality than we now have. Modern evidence suggests that there has been a progressive increase in seasonality and aridity during the last 500,000 yrs culminating in our modern highly seasonal climates (Schultz et al., 1972). This change caused the complex Pleistocene biomes to break up and destroyed coevolved systems among plants and animals. The modern distributions of plants and animals were achieved at this time, and steppes and deserts first appeared. The reduction in forests at this time may have been partially the result of an increase in the significance of fire due to newly seasonal climates. These major environmental changes are associated with an important megafauna extinction similar in scope to extinctions that occurred during the Tertiary (Martin, 1985); the Tertiary extinctions may also have been associated with periods of increased seasonality. All of these environmental changes must have had profound effects on humans, and the rapid cultural evolution of the Holocene may be in part due to the inception of increased climatic seasonality.


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Web version updated March 25, 2010. Original publication date 1987.