You and an
oak tree have something in common: you're both big. Unlike viruses
and bacteria, you and an oak tree are both made up of trillions of
cells. There's something else you and an oak tree have in common:
you both began as an individual cell, which then divided again and
again, its daughter cells differentiating along the way to produce
tissues. In your case, they turned into bone, muscle, liver, and
such. In the oak's case, the cells became bark, leaf, root.
You and an oak tree have a third thing in common: you evolved
from single-celled ancestors. By analyzing DNA from a wide range of
species, scientists have been closing in on the closest microbe
relatives of plants and animals in recent years. For animals, the
evidence is pointing to a protozoan called a choanoflagellate. For
plants,
a few lineages of green algae are emerging as the closest
relatives.
Once scientists find out who these relatives are, they can then
tackle the puzzle of how big things evolved from small ones. How did
multicellular animals evolve the genes they use to develop from
ancestors that didn't grow bodies? Part of the answer, it turns out,
is that at least some of the genes were already there in the
single-celled ancestors.
A new study takes a look at the evolution of plants. Land plants
evolved from algae over 475 million years ago according to the
fossil record. The earliest plants resembled today's liverworts and
mosses--which, not coincidentally, belong to the oldest branching of
living plants. While moss and oaks may not seem to have much in
common, they share a very important trait. They each produce cells
that can divide asymmetrically, forming a shoot that can rise
upward. This organ, known as a meristem, allows land plants to be
more than a sheet of slimy algae. They can be three-dimensional.
Becoming 3-D brought many benefits to early land plants, letting
them rise up out of water, for example, while reducing their surface
area-to-volume ration to cut down on their water loss.
In 2003 French scientists reported
the discovery of body-building genes in moss. When moss develop
buds, their cells use six related genes, which the scientists named
BIP1 through BIP6. None of the BIP genes were active in moss cells
before budding, and when BIP genes are knocked out, moss cells can't
form three-dimensional structures.
Canadian researchers have now looked for similar BIP genes in
other species. They've found them in land plants, such as corn and
cress, which supports the idea that these genes have played a
crucial role in the growth of land plants since they first became
three-dimensional. The scientists don't find the genes outside the
land plants--except for a species of green algae, Mesostigma viride.
It turns out that Mesostigma is also among the closest relatives of
land plants. (The paper
is in press at Molecular Biology and Evolution; you can get the pdf
for free on the lead author's publications
page.)
Mesostigma is a humble little alga, and so it obviously doesn't
use its BIP genes to grow into towering trees. Exactly what it does
with the genes remains to be discovered--after all, the genes
themselves have only just been discovered. But the most logical way
to interpret the new findings is that the single-celled ancestors of
land plants had BIP genes long before they co-opted them for growing
3-D.
The scientists point out that both Mesostigma and its land plant
relatives have many copies of BIP. That's a common
pattern in evolution. Genes get accidentally duplicated, and
natural selection sometimes favors having lots of extra copies
around. As green algae first shifted from the ocean to fresh water,
duplicated BIP genes may have been favored to help them adapt to
their new home. Later, land plants evolved in one lineage of these
green algae, and they co-opted one or more BIP genes to grow
three-dimensionally. Land plants evolved still more copies of BIP
for growth, and then in later lineages, some of these genes were
co-opted yet again for new uses.
This pattern of duplication and co-option turns up again and
again, from microbes to humans. It even turns up in another
small-to-big transition: the evolution of animals. Among the tools
animals use to grow multicellular bodies, they need communications
equipment so that neighboring cells can talk to one another and
coordinate their growth. Choanoflagellates, the closest
single-celled ancestors to animals, have the
same genes that animals use for this cross-talk. In other words,
the single-celled ancestors of animals already had some of the
equipment their descendants would use to build big bodies.
Just another item to add to the list of things you and an oak
tree have in common.
Update 2/20 3 pm: changed 475 years to 475 million years.
What's a million between friends? (Thanks to Gal Haspel.)
1. Carl Manaster on February 20, 2006 01:00 PM writes...
I'm pretty sure that the historical record can confirm the fossil record here; I believe I have read about land plants in books that date considerably before 1531 AD. :-)
Permalink to Comment2. john on February 20, 2006 08:33 PM writes...
Another thing humans have in common with them is that heretics were traditionally burned at the stake, built often from oak.
But seriously, how could the first land plants have evolved 475 million years ago if the planet has only been here for thousands of years?
No, really... If the mighty oak tree evolved from moss, why do we still have moss.
Permalink to CommentOK... I can't be serious today. Hey, it's a holiday!
3. Peter Ellis on February 21, 2006 04:47 AM writes...
Is it completely clear that the BIP genes arose before the evolution of 3D plants? Could it also be that Mesostigma has 3D plants in its ancestry but has somehow lost the ability to form meristem and thus returned to algal life?
Permalink to Comment4. Carl Zimmer on February 21, 2006 09:33 AM writes...
Peter (#3): If Mesostigma did indeed descend from a land plant and lost its ability to make meristems, then you'd expect that one particular lineage of living land plants would be more closely related to it than to other land plants. Consider a parallel example: whales lost their legs, having descended from mammals with legs. Studies on DNA bear this out: hippos are more closely related to whales than to other mammals.
But when it comes to Mesostigma, studies on plant DNA don't bear this prediction out. While the precise placement of Mesostigma is still being worked out, it's now clear that all living land plants are more closely related to one another than they are to Mesostigma. So it branched off before the origin of land plants. Here's one recent study bearing on ths.
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