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in Acre (Brazil). However, the major fraction of these fossils was very fragmented and their
exact relationship with
Inia
is difficult to establish. Fittkau (1974) and Grabert (1967, 1983,
1984 a, b, c) postulated that the Iquitos-Purús arch divided the Amazon basin into two flow
water directions, one to the Atlantic and other to the Pacific, during the Middle Miocene. The
most extended hypothesis for the
Inia
origin is that proposed by Grabert (1984 a, b, c) and
related with the Iquitos gate. This author stated some Iniidae, such as
Proinia patagonica
(although other authors like Barnes negated the relationship of this form with the Iniidae),
inhabited the Pacific coastal region in the middle Miocene (15 million years ago) and
penetrated to the molasse lakes which were forming when the Andean orogenesis began. With
the completion of the Andean orogenesis, five million of years ago, the link between these
molasse lakes (sub-Andean freshwater molasses) and the Pacific Ocean disappeared. The
Iniidae forms could have penetrated to the sub-Andean freshwater molasses by way of
Guayaquil Bay or through the Arica entrance, later, during the Pleistocene, when Beni lake
was extinct. This is the area in the current Bolivian Amazon where
Inia boliviensis
is found.
The surrounded areas of this sub-Andean freshwater lake system were savannas and arid
regions and rapidly growing Andean cordilleras, where the turbidity of waters was extremely
high. Following Grabert (1984a, b, c), at this moment, was the appearance of the most
ancestral form of
Inia
,
Inia boliviensis
, characterized by the presence of microphthalmia. This
author sustained that
Inia boliviensis
was the first form of
Inia
because it has a larger number
of teeth (24 more teeth) and a smaller brain capacity (100 cc lesser) than the
Inia
's forms
from the Amazon and Orinoco (
Inia geoffrensis
). Thus,
Inia boliviensis
originated five
million of years ago. Later during the late Pliocene or beginning of the Pleistocene (about 1.8
millions of years ago) throughout the Purus or the Iquitos gates, this
Inia
form migrated into
the Amazon basin giving rise to
Inia geoffrensis
. Such as the Amazon basin has more
biotopes and more diverse ecological conditions than the original sub-Andean freshwater lake
system, the ―new‖
Inia
(
Inia geoffrensis geoffrensis)
form had better cerebralisation (because
the species having an increased ultrasound capacity making is more efficient than all of the
new biotopes that colonized) and the reduction of the dental count could be related to a better
capacity to obtain a major quantity of fish species. Later, much more recently (10,000 years
ago during the Holocene), with the increase of humidity, rain regimes and sea levels, the
blackwater rivers (such as the Negro River) and flooded forest (várzea) formed. The
aforementioned author considered that the formation of these black-water rivers between the
Amazon and Orinoco basins was the main process that originated a different subspecies in the
last basin,
Inia geoffrensis humboldtiana
, because black waters, with their high acidity and
the lack of trophic resources, could be a barrier for
Inia
. However, the molecular genetics
results herein were not in agreement with the Grabert's hypothesis. Clearly, the following
insights contradicted this hypothesis:
1.
The microsatellite analyses showed some historical demographic changes, and
presented clear evidence that the Amazonian population expansion process occurred
around 35,000 years ago (Kimmel et al.,' test) or around 9,000-76,000 years ago
(Zhivotovsky et al.,' test, in the most favorable circumstances for this procedure).
The mtDNA analysis showed that the two main Amazon haplotypes diverged about
23,000 years ago. Henceforth, the Amazonian expansion for
Inia
could not be 1.8
millions of years ago, such as was claimed by Grabert (1984a, b, c) or about 1.5
millions of years ago during the first Ice Age (Nebraska or Günz) as it was defended
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