The Economist - USA (2021-02-06)

66 Science & technology The Economist February 6th 2021

plastic objects. Dr Peng himself was on

board for three of these trips.

It was a risky business. Submerged jun-

kyards of this sort often contain ropes and

fishing nets that can entangle a subma-

rine’s propellers. Paradoxically, that was

why the researchers chose to visit in per-

son, rather than sending a robot. People on

board are able to steer and avoid these haz-

ards more easily than a crew on a support

ship who are relying on cameras to see

what is happening.

To ensure their craft’s safety, the re-

searchers collected objects only from the

edge of each dump, and also collected

some by dredging. In all, they obtained 33.

Most were bags, bottles and food wrappers,

but they picked up some derelict fishing

ropes and traps as well.

As Dr Song and Dr Peng had hoped,

these objects were teeming with life. When

they examined their finds in a laboratory,

they found nearly 1,200 individual orga-

nisms representing 49 species of crusta-

ceans, corals, echinoderms, flatworms,

molluscs (see picture), polychaete worms

and fungi. They also discovered evidence

that some of these species were breeding.

There were egg capsules from four differ-

ent types of snail, and a cocoon from a flat-

worm known for parasitising crustaceans.

This result suggests that accumulations

of plastic are, indeed, benthic oases. Data

from such depths are sparse. But 49 species

is more than the 41 found in 2016 on a dead

whale at the bottom of the Atlantic Ocean.

As to why organisms colonise the ob-

jects in these accumulations, the short an-

swer is, “because they are there”. Most of

the deep-ocean floor is covered with ooze,

on which it is difficult to get a purchase ex-

cept by burrowing. Solid objects suitable

for settling on are at a premium. Whether

it is a good or a bad thing that people are

adding to the number of these objects, and

thus to the richness of the benthic fauna, is

a debatable point. What is not in doubt,

though, is that even Earth’s remotest hab-

itats cannot escape human influence. 

Denizens of the deep

A

bout370m years ago, in the latter

part of the Devonian period, the

ancestor of all land vertebrates stepped

out of the ocean and began to take ad-

vantage of the untapped riches found

ashore. This was a big step, both literally

and metaphorically, and evolutionary

biologists have long assumed that bring-

ing about the anatomical shift from

functional fin to proto-leg which enabled

it to happen required a fortuitous coinci-

dence of several genetic mutations. This,

though, may not be the case. A paper just

published in Cell, by Brent Hawkins,

Katrin Henke and Matthew Harris of

Harvard University, suggests the process

was propelled by a single genetic change

of the smallest sort possible.

The better to understand the origin of

tetrapods, as land vertebrates are known

collectively to zoologists, the trio were

looking at what happened to zebrafish (a

common subject of experiments in

developmental biology because they are

small, transparent and breed prolifically)

when they made minor tweaks to those

fishes’ genes. Searching through more

than 10,000 mutated specimens they

noticed that one group of mutants sport-

ed an unusual pattern of bones in their

pectoral fins. Instead of having four, they

had six.

Intriguingly, the additional pairs were

some distance from the body, and the

bones involved lay parallel with each

other in the way that the radius and ulna

do in the forelimb of a tetrapod (see

diagram below). Moreover, and yet more

intriguingly, the two new bones in-

tegrated neatly with the fin’s muscles

and articulated well with the rest of the

local skeleton. Most intriguingly of all,

however, was that this considerable

anatomical shift was brought about by

the substitution in a single type of pro-

tein molecule, called Wasl, of a single

one of its amino-acid building blocks.

Wasl is a signalling protein. But it is

not one which, as far as the team could

tell by searching through the literature

on embryonic development, had previ-

ously been associated by anyone with the

process of limb formation in vertebrates.

However, an experiment they then con-

ducted on mice, which involved knock-

ing out the gene that encodes Wasl,

resulted in deformation of the pertinent

bones in all four of the rodents’ limbs,

not just the forelimbs. Clearly, then, this

protein does indeed play a role in tetra-

pod limb formation.

The most recent common ancestor of

zebrafish and mice predates even the

Devonian. That gives lots of time for

patterns of embryonic development to

have changed in the lines leading to

those two species—and, specifically, to

have changed in the way that the fins of

modern fish develop. So the fact that

nowadays the mutation the team have

discovered affects only the pectoral fin

does not rule out the possibility of its

having also stimulated, way back then,

the arrival in the pelvic fin of the fishy

progenitor of the mouse, of the bones

now known as the fibula and tibia. It

therefore looks quite possible that Drs

Hawkins, Henke and Harris have found

the source of the crucial change that

enabled the ancestor of mice—and of

human beings, too—to scramble ashore

and leave the sea behind.

The origin of land animals

Getting a leg up

A tiny genetic alteration may have let vertebrates leave the sea

Changing its stripes

Source: “Latent developmental potential to form limb-like skeletal structures in zebrafish”, M. B. Hawkins et al., Cell, 2021

A zebrafish mutant develops limb-like bones in its fins

Human

arm

Normal

zebrafish

Mutantversion

Developsnewbonesthat

arenotdirectlyattached

totheshoulder.Theyform

jointswithneighbouring

bonesandareintegrated

into the musculature.

Pectoral fin

Fin rays

Bones attached

to shoulder

New

bones