When Did The Immune System Evolve?
The major clue is that the human-style immune system exists in all
of the jawed vertebrates. But,
it doesn't exist in the jawless vertebrates such as the hagfish and
lamprey. And, it doesn't exist in any invertebrates such as starfish
and jellyfish.
(To be more precise: we've measured some number of species, and
that is the pattern we've seen so far. It is therefore a prediction of
the Theory of Common Descent that we
will continue to see this pattern as we measure more species.)
According to the Tree of
Life project, the tree of descent from the the first vertebrate
was:
============================== Hyperoartia (lampreys)
|
| ?== Euconodonta ![[Extinct]](extinct.gif)
|
| ====================== Pteraspidomorphi
| |
| | ?|| Thelodonti
| |
=====| | = ?== Anaspida
| | |
====1===| | =========== Galeaspida
| | |
======| | = ?== Pituriaspida
==2==| |
==3==|===== Osteostraci
|
===4== Gnathostomata (jawed vertebrates)
Some of the groups are known from very few (or very partial) fossils,
hence the question marks. The main fossil characteristics making us
choose this particular tree are:
- Node 1: Mineralized exoskeleton, sensory-line canals
and grooves
- Node 2: Perichondral bone or calcification,
externally open endolymphatic duct
- Node 3: Paired fins containing musculature and
concentrated in pectoral position, two dorsal fins, epicercal
(i.e. upwardly tappering) tail, sclerotic ring and scleral
ossification, cellular dermal bone
"Dermal bone" meaning, of course, a jaw.
Unfortunately, most of these groups are extinct and it's too late
to measure their immune systems. The best we can do is conclude that
the human-style immune system arose somewhere along the path 1-2-3-4.
Geological dating of the fossils gives us a window about 100
million years long, with the possible dates falling in the Ordovician,
Silurian and Devonian Periods. Science journalists have settled on
"450 million years ago" as the simplified answer.
Last modified: 15 April 1999
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