Common Ancestry
All organisms on the earth are related. Some are more closely related than others. It has long been a hoped
that scientists could determine dates for the branching points in the evolutionary tree of life. This dating has
more or less been accomplished thanks to new molecular techniques and we can finally assign time periods
to our line of descent from, not only simpler animals, but from plants, fungi and bacteria. Only since the late
1900’s have we been able to squeeze evidence about our relatedness with other animals out of genes.
Before this time the best way that we could draw inferences about our zoological family tree was to compare
skeletons and tissues from different species looking for the kind of changes that take lots of time to occur.
Comparing modern animal skeletons with chemically or radiometrically dated fossil skeletons gives us a lot
of information about how species changed with geological time. For instance, if we knew that fossil
amphibians first showed up in the fossil record about 350 million years ago then we could roughly infer that
we shared a common ancestor with the remaining amphibians today, about that long ago. This means that
our line was amphibious, that we laid eggs and were born as tadpoles about that long ago. Of course this
inference is predicated on the notion that we evolved from amphibians which, after a close look at the natural
record, is patently obvious. If it was difficult to determine when a certain order of animal appeared on earth
from the fossil evidence we could compare its anatomical traits to other, seemingly, closely related animals
to devise a family tree based on similarities. This technique, called comparative anatomy, gave scientists a
number of pretty accurate, but largely imprecise, estimates as to how many years have passed since
animals from one line diverged from animals of another. Telling us when amphibians became distinct from
fish, when reptiles became distinct from amphibians… and when we became distinct from the other apes.
Today we geneticists use a much more accurate method to determine relatedness- they measure the
number of molecular differences in the DNA between species. When a geneticist tells us that we are more
than 98 percent chimpanzee, that means that if we lined a matching strand of our DNA right up next to theirs,
less than 2% of the molecules would be non-identical, and the rest would match our sequence exactly. We
can make these comparisons with all animals, and similar comparisons with all forms of life for that matter, to
determine both interrelatedness and time since divergence. This way of estimating the branches for life’s
family tree is pretty precise with closely related organisms, but gets messier the farther you branch out. Our
divergence from plants, is estimated to have taken place sometime around 1.25 billions years ago give or
take a few tens of millions of years. Our divergence from LUCA, the last universal common ancestor, would
have taken place just after the first organism on the earth began to diversify well over 2 billion years ago.
Happily, this 2 billion year estimate that we get when comparing our DNA to the most archaic bacteria
matches the dates for the first evidence of bacterial life on earth.
Unfortunately for dinosaurs and trilobytes, this technique really only works for extant, living animals because
you need to get DNA in order to make this comparison. In other words, we can’t compare our DNA to that
found in most fossils because their DNA has broken to pieces. Excitingly though, some relatively new fossils,
found in the last 30 to 50,000 years can yield DNA though. This means that we can compare our DNA to
recently extinct human ancestors.
The following list gives dates for our divergence from other living groups of animals. The first entry on the list
indicates that we shared a common ancestor with chimpanzees, our closest living relative, only 6 millions
years ago. This does not necessarily mean that we were chimpanzee at that time, just that humans and
chimpanzees were members of the same breeding group of individuals and had not split apart and evolved
into different species.
6 mya Chimpanzees
7 mya Gorillas
14 mya Orangutans
18 mya Gibbons
25 mya Old World Monkeys (colobus, langur, proboscis, guenon, macaque, baboon)
40 mya New World Monkeys (spider, howler, woolly, capuchin, squirrel, marmoset)
58 mya Tarsiers
63 mya Prosimians: Lemurs (lorises, bushbabies, pottos, aye-ayes)
70 Tree Shrews and Colugos
75 mya Rodents and Rabbits and Hares (pikas, hares, rabbits, mice gerbils, voles, lemmings, hamsters,
squirrels, porcupines)
85 mya Laurasiatheres (shrews, hedgehogs, moles, bats, hippos, whales, camels, pigs, deer, sheep,
horses, rhinos, cats, dogs, seals, walruses)
95 mya Xenarthrans (sloths, anteaters, armadillos)
105 mya Afrotheres (elephants, manatees, moles, shrews, aardvarks, dugongs, hyraxes)
140 mya Marsupials (opossums, bandicoots, moles, wombats, kangaroos, possums, koalas, Tasmanian
devils, numbats, marsupial moles)
180 mya Monotremes (echidnas, platypus)
310 mya Sauropsids (turtles, lizards, snakes, crocodiles, birds, tortoises, iguanas, geckos, skinks, ostriches,
waterfoul, passarines, perching birds)
340 mya Amphibeans (salamanders, frogs, toads, caecilians)
417 mya Lungfish
425 mya Coelacanths
440 mya Ray-finned Fish
460 mya Sharks, Chimaeras, Rays and Skates
550 mya Lampreys and Hagfish
575 mya Lancelets
580 mya Sea Squirts and Salps
570 mya Ambulacrarians (sea urchins, sanddollars, sea cucumbers, starfish, brittle stars,)
590 mya Protostomes (mollusks, insects and some worms)
630 mya Acoelomorph Flatworms
650 mya Cnidarians (jellyfish, corals, anemones)
700 mya Ctenophores (combjellies)
750 mya Placozoans
800 mya Sponges
900 mya Choanoflagellates
1 bya Drips
1.1 bya Fungi (mushrooms, yeast, rusts, morles, truffles)
1.2 bya Amoebozoans
1.25 bya Plants
2 bya Archaea Bacteria
2.? bya Eubacteria
| Organization for the Advancement of Interdisciplinary Learning |