Mesa Schumacher
Creating a ‘Humanzee’ – science or fiction?
Written by Stefanie Schuster
Cross-breeding between species has long been reported for horses and donkeys, which gives rise
to mules and more recently for tigers and lions (‘Liger’) and bovines and antelopes (‘Beefelo’), but is
cross-breeding between great apes such as chimpanzees and humans possible? Is the link between
the two species close enough? What actually determines the possibility of cross-breeding? Is it
the similarity of DNA between species or the number of chromosomes, or is it rather dependent on
evolutionary history?
Great apes are obviously different in
body posture, strength, communica-
tion, behavior and (in most cases,
according to human assessment) in-
telligence, but more than 95% of their
whole DNA sequence is identical to
ours, with even 98.8% similarity for
their coding DNA (1). This is definitely
more than other pairs of species that
can produce offspring, like horses and
donkeys that have an estimated simi-
larity of 94%, so it surely cannot be the
answer to the problem.
A major factor in determining the pos-
A hybrid cross between a male lion and a
female tiger: the liger. Source: M. Sullivan
(Flickr.com)
26 | NEUROMAG |May 2017
sibility of fertilizing another species’
eggs is the amount of time that has
elapsed since the last individual that
both species descend from, referred
to as the last common ancestor. While
the last common ancestor of humans
and chimpanzees is often dated back
to 13 million years ago, a recent anal-
ysis observed more recent gene flow
in chromosomes: humans and chim-
panzees still had sex and produced
offspring until ‘as recent’ as 4 million
years ago (2). This is similar to the dat-
ing of the last common ancestor of
modern horses and donkeys which is
dated at 4 to 4.5 million years ago (3).
So, it appears that it is more compli-
cated than a simple date of last ‘gene
distribution’.
During my research, I found many
people arguing that chimpanzees and
humans can surely not produce viable
offspring due to the different number
of chromosomes, namely 48 chromo-
somes for chimpanzees and 46 chro-
mosomes for humans. This is simply
not true. Again, my example is based
on horses and donkeys (but it is also
true for bulls and antelopes): they can
successfully crossbreed, even though
their number of chromosomes is dif-
ferent (62 and 64 chromosomes for
horses and donkeys, respectively). The
offspring with its 63 chromosomes
is, however, infertile. Whether this is
caused by the uneven number of chro-
mosomes as this lonely number 63
cannot pair with another chromosome
and segregate correctly or rather by
an effect more downstream of pairing,
namely the stop of spermatogenesis
at a certain point with unknown rea-
sons is still under debate.
You might say to yourself that this
issue of cross-breeding is just a big
thought experiment and while we can
continue to pontificate until the cows
come home we’ll never be able to test
it. This type of research is of course
precluded for ethical considerations
in our current times, but in the 1920s,
the Russian biologist Ilya Ivanovich