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From: flinn@seismo.UUCP (E. A. Flinn)
Newsgroups: net.origins,net.bio
Subject: An acquatic phase in human evolution?
Message-ID: <1143@seismo.UUCP>
Date: Thu, 26-Apr-84 10:32:42 EST
Article-I.D.: seismo.1143
Posted: Thu Apr 26 10:32:42 1984
Date-Received: Fri, 27-Apr-84 04:42:53 EST
Distribution: net
Organization: Center for Seismic Studies, Arlington, VA
Lines: 76

There is an interesting article by Elaine Morgan on pages 11-13 of the
April 12th 'New Scientist' about the hypothesis that there may have
been an acquatic phase in human evolution.  She notes Alister Hardy's
suggestion in 1960 that such a phase could account for several
characteristics that are unique to humans among the hominids, but
common among acquatic mammals (lack of body hair, presence of
subcutaneous fat, face-to-face copulation, the position of fetal hair,
and weeping).  It would also explain the pre-adaptations necessary for
the emergence of bipedalism and speech.  There was flooding in
northeastern Africa during the "fossil gap" 4-8 million years ago,
after which bipedal hominids first appeared; the flooding may have
induced isolated populations of apes to adopt an aquatic style of life,
and the subsiding waters would have forced these apes back to land.

	Another way that humans are unique among land mammals is in
our sodium homeostasis: other mammals suffering from sodium deficiency
have a compelling craving for salt, and once the deficiency is made
up, the appetite vanishes sharply.  This mechanism does not operate in
humans, so that salt intake has little or no relation to the body's
needs, varying from salt aversion in some peoples to excessive
intake in western countries.  High death rates from diarrhoeal
infections have been reported in underdeveloped areas where salt is
readily available and its ingestion would save lives, but the active
craving triggered in other species by sodium deficit is not triggered.

	Alone among primates, humans cool their bodies by sweating,
which is profligate of sodium as well as water.  It's hard to escape
the conclusion that one single species of ape in Africa revolutionized
its management of both intake and excretion of salt; apparently it is
difficult to explain this without humans having spent some of their
evolution time in the sea.  Marine birds, for example, allowed for
excessive salt intake and evolved new ways to eliminate the surplus;
humans are apparently the only land animals to have done the same.

	Humans can hold their breaths for long periods while diving;
again, this is unique among land mammals.  In humans, as in seals and
ducks, vascular constriction during diving is not limited to the
arterioles but extends to larger arteries also, which indicates
specialized adaptation.  Studies have shown that the human diving
responses such as apnoea and bradycardia are not purely unconditioned
reflexes, but are influenced by higher brain centers to the extent
that they can develop in anticipation of immersion - dolphins also
have this characteristic.  Morgan notes that an incidental advantage
arising from this characteristic is the acquisition of sophisticated
volitional breath control, without which human speech was unlikely to
have evolved.

	As to the nakedness of the naked ape, there are only two
environments known to be conducive to hairlessness in mammals: aquatic
and subterranean.  Only one environment is known to be conducive to
hairlessness plus the subcutaneous fat layer that characterizes humans
(alone among primates): the acquatic.

	The Australopithecus afarensis skeleton shows that this ape was
bipedal, and that the bipedalism was so far advanced that this must have
evolved earlier, in the fossil gap.  A. afarensis left no evidence that
they were hunters, so the inference is that the bipedalism evolved for
some purpose other than carrying weapons and hurling spears.  While a
chimpanzee's pelvis is narrow and long, Lucy's was wide and short (as
in Homo sapiens), indicating long habituation to a mode of locomotion
in which the spine and the hind limbs are roughly in the same plane,
instead of at an angle of about 90o, as in most quadrupeds.  This
alignment is unknown in tree-dwelling primates.  Swimming is the only
known method of mammalian locomotion in which this pelvic orientation
can evolve without necessitating simultaneous weight-bearing
adaptations in the lower limbs.  If Lucy's ancestors had been swimmers,
their descendants returning to land would have had a pelvis powerfully
pre-adapted to bipedalism, but their legs and feet would not yet have
been fully adapted.  This too is seen in the Lucy skeleton, since the
toes are long, slender, and curved, the big toe is still slightly
opposable, and the automatic locking mechanism of the knee was not yet
perfected.

	Morgan discusses other points, and refers to her book, 'The
Acquatic Ape,' published in 1982.