Ever since 1931, when we found remains of crocodiles and fish at several sites, we have known that a non-mammalian fauna was present in the Olduvai deposits, but it is only during the past three years that any considerable quantity of fossils of this natuie has been found in situ.

It has not been possible to obtain detailed reports upon any of the non-mammalian fossils in time for inclusion in this volume. The following summary, however, indicates the range of material. The following categories have been found:

1. Mollusca

2. Pisces

3. Amphibia

4. Reptilia

5. Aves

MOLLUSCA

Fossil mollusca occur in Beds I, II and IV. In Bed IV the commonest are those representing Unio sp. In Beds I and II there is a small bivalve akin to Corbicula, as well as specimens allied to Melanoides tuberculata.

PISCES

Fossil fish remains occur in Beds I, II and IV. Some are found in sands and silts, where they became imbedded in lake deposits at death. The majority of the specimens, however, come from the living-floors of early man and were clearly part of his diet.

The presence of such fish as Tilapia and Cichlidae in certain deposits suggests that at these periods the lake contained a considerable amount of water.

So far, only a brief preliminary report on the fish remains is available, based on itiaterial from two living-sites in Bed I, two in Bed II, and various sites in Bed IV.

ABSTRACT

Bed I at Olduvai Gorge is a conformable sequence of lava flows and varied sedimentary deposits that extend upward from a welded tuff overlying the Precambrian basement to the top of a widespread marker bed. Bed II is a sequence of lacustrine clays and laterally equivalent fluvial, eolian and pyroclastic deposits. Bed III comprises alluvial deposits and a laterally equivalent assemblage of fluvial, lacustrine and eolian beds. Bed IV can be widely subdivided into a lower unit of fluvial clays, sandstones and conglomerates and an upper unit of eolian tuffs. Although moister than that of the present day, the climate was relatively dry throughout much of the time that these beds were deposited. Semi-desert or desert conditions rather like those of the present day may have prevailed at least twice. Tectonic movement seems to have taken place between the deposition of Beds III and IV.

A stratigraphic and environmental framework more detailed than that of Reck (1951) and Pickering (1960) is here presented for the Pleistocene succession of Olduvai Gorge (Fig. 1), which contains hominid remains of great antiquity (Leakey, Curtis and Evernden, 1961) and an unsurpassed sequence of Palaeolithic culture levels (Leakey, 1951; 1963). The need for geological information about the Olduvai succession became clear in recent controversies over the potassium–argon dates and geologic histories of Beds I and II (Straus and Hunt, 1962; Von Koenigswald, Gentner and Lippold, 1961; Leakey, Curtis and Evernden, 1962).

In this first volume I have tried to set out the present state of our knowledge of the background evidence, against which the very important fossil human remains from Olduvai and the Stone Age cultural sequence there can be assessed. Preliminary reports upon the fossil hominid discoveries have appeared from time to time since 1958, both in Nature and in the popular press, and scientists are therefore already aware of the general picture. The study of the fossil human remains and of the stone tools, together with a detailed report upon the excavations of sites will appear in subsequent volumes.

Volume 2 by Professor Phillip V. Tobias, of the Medical School, Johannesburg, is now in the final stages of preparation, and will be published during 1967. It deals with the skull of Zinjanthropus boisei in very considerable detail.

Three further volumes are in preparation. Mrs M. D. Leakey's deals with the living sites and cultural sequence of Bed I and Bed II, and will include chapters by Dr Leakey on the fauna associated with each living floor.

Dr Richard Hay's volume, dealing with the details of the geology of Olduvai Gorge, is being prepared.

Another volume by Professor Phillip Tobias, also now in active preparation, will be a careful study of Homo habilis, the LLK skull, and a variety of other Olduvai hominids.

At the time of writing this report, i.e. up to the end of December 1961, fossil hominid remains had been found at three different levels in Bed I.

In the chapter which Dr A. T. Hopwood wrote for my Olduvai book, published in 1951, he dealt with certain aspects of the fauna of Olduvai. He gave his interpretation of the fossil evidence then available in respect of the age of Beds I to IV. He concluded that these four beds formed ‘a single unit’ and that, in his own words, ‘there is no faunistic evidence to suggest that the lower part of the Olduvai series is of Lower Pleistocene age’.

In 1935 (Leakey, 1935) I had published the view that Bed I of Olduvai was of Lower Pleistocene age; but because of Dr Hopwood's categorical statement, based upon his study of the fauna, I later retracted this opinion, and in the 1951 book I accepted his suggestion and treated the whole of the main Olduvai sequence as belonging to the Middle Pleistocene. In spite of this, however, Dr D. G. Maclnnes and I felt constrained to point out that we considered the break between Beds I and II and Bed IV was more significant than Dr Hopwood had suggested. However, we accepted the fossils of Beds I and II as representing a single faunal unit, for we did not, in those days, see any reason to challenge the identifications which were given in Dr Hopwood's list.

In the first chapter of my earlier book (Leakey, 1951) on Olduvai Gorge and on the evolution of the Hand-axe culture there, I outlined the sequence of events which led to my first expedition to Olduvai in 1931. I also summarised the work carried out from 1931 to 1947. There is no need to refer here to any of that earlier part of the story, since it can easily be found in the first book and I shall discuss the sequence of the events from 1947 onwards.

In 1947 the first Pan-African Congress of Prehistory met in Nairobi and at its conclusion I arranged an excursion to sites in Tanganyika, including Olduvai Gorge. One of the first places to which I took the visiting scientists on that occasion was FLK I, for it was there, during the 1931-2 season, that we had first proved the existence of a primitive Stone Age culture in situ in Bed I. This is the culture which I later described under the name of Oldowan and which has sometimes been erroneously referred to as the ‘Pebble culture’. Other parts of the Gorge were, of course, also visited, but both on that occasion and on many others, from 1947 to 1958, I took visiting scientists to see the site FLK I. Thus the place which eventually gave us such a wealth of new information about earliest man and the evolution of the Oldowan culture is one which many leading Prehistorians and Pleistocene geologists had seen, before these discoveries were made.